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U  (^  S  /^     3  3 


COLUMBIA    UNIVERSITY  BIOLOGICAL   SERIES.     IV. 


THE   CELL 


IN 


Development  and  Inheritance 


BY 


EDMUND    B.  WILSON,   Ph.D. 

PROFESSOR  OF  INVERTEBRATE  ZOOLOGY,    COLUMBIA   UNIVERSITY 


"  Natura  nusquam  magis  est  tota  quam  in  minimis  " 

PLINY 


THE    MACMILLAN    COMPANY 

LONDON :  MACMILLAN  &  CO.,  Ltd. 
1896 

All  rigftJs  reserved 


Copyright,  1896, 
By  the  MACMILLAN   COMPANY. 


NorbJooI)  i^ress 

J.  S.  Cushing  &  Co.  -  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


ERRATA 


P.  27,  1.  15  :  /or  revolves  read  resolves. 

P.  29,  1.  9  :  /or  nucleo-proteids  read  nucleo-albumins. 

P.  44,  1.  25  :  /or  Zimmerman  read  Zimmermann. 

P.  76,  1.  I :  /or  mable-fibres  read  mantle-fibres. 

P.  78,  1.^0:  /or  '72  read  '82. 

P.  130,  second  line  from  bottom:  /or  CEdigonium  r^a^ CEdogonium. 

P.  160,  in  explanation  of  cut:  /or  Pibularia  read  Pilularia. 

P.  162,  1.  2:  /or  Brogniard  r^fa^/ Brongniart. 

P.  196,  1.  21 :  /or  four  read  eight;   1.  22 :  /or  eight  read  four. 

P.  199,  1.  21 :  /or  dermids  read  desmids. 

P.  218,  1.  2 :  /or  69  read  24. 

P.  222,  I.  7 :  /or  92,  A  read  92,  B. 

P.  235,  1.  16:  /or  Strongylocentrotus  read  Sphserechinns. 

P.  252,  1.  I :  /or  GEdigonium  read  CEdogomnm. 

P.  309,  1.  3 :  add  and  by  Herlitzka  (^95)  in  Triton. 

P.  348,  1.  31  :  /or  Camptocanthus  r<?a^  Canthocamptus. 

P-  353.  1-  9  ••  /or  XXXVII.  read  XXVII. 

ADDENDA 

(The  following  notes  refer  to  an  Appendix,  inserted  after  p.  357,  in  which  are  contained 
various  addenda  and  critical  notes.  Most  of  these  refer  to  works  that  have  appeared  since 
the  first  part  of  this  edition  was  issued.) 

P.  15.     Double  centrosomes  in  resting  cells.     Note  i. 

P.  16.     Continuity  of  linin-network  and  cyto-reticulum.     Note  2. 

P.  19.     Protoplasmic  structure.     Note  3. 

P.  27, 1.  I.     Size-relations  of  nucleus  and  cytoplasm.     Note  4. 

P.  27, 1.  33.     Chromatin-granules.     Note  5. 

P.  40.     Position  of  centrosome  in  epithelial  cells.     Note  6. 

P.  43.     Protoplasmic  continuity.     Note  7. 

P.  44.  Add  to  Literature  List,  A.  Zimmermann  :  —  Die  Morphologic  und  Physiologic 
des  Pflanzlichen  Zellkernes;  Jena,  Fischer,  1896. 

P.  53.       Origin  of  the  mitotic  figure.     Note  8. 

P.  82.       Connections  between  attraction-spheres.     Note  9. 

P.  112.     Nutrition  of  the  ovum.     Note  10. 

P.  126.     Formation  of  the  spermatozoon.     Note  11. 

P.  156.     Van  Beneden's  A^iews  on  the  centrosome  in  fertilization.     Note  12. 

P.  159.     Lillie  on  the  centrosomes  in  Unio.     Note  13. 

P.  159.     Hertwig  on  unfertilized  sea-urchin  eggs.     Note  14. 

P.  192.     Reduction  in  vertebrates.     Note  15. 

P.  192.     Origin  and  meaning  of  tetrads.     Note  16. 

P.  194.     Reduction  in  vertebrates.     Note  17. 

P.  197.     Tetrads  in  ferns.     Note  18. 

P.  199.     Reduction  in  diatoms.     Note  19. 

P.  200.     Reduction  in  insects.     Note  20. 

P.  226.     Doubts  regarding  genetic  continuity  of  the  centrosome.     Note  21. 

P.  228.     Centrosome  in  echinoderms.     Note  22. 

P.  240.     Chemistry  of  nucleic  acid.     Note  23. 

P.  256.     Ingestion  of  food-granules  by  nucleus.     Note  24 

P.  278.     Laws  of  cleavage.     Note  25. 

P.  292.     Chemical  stimuli  to  cell-division.     Note  26. 

P.  313.     Hertwig's  theory  of  development.     Note  27. 

P.  329.     Regeneration  of  the  lens.     Note  28. 


Co  mg  frientJ 
THEODOR    BOVERI 


PREFACE 

This  volume  is  the  outcome  of  a  course  of  lectures,  delivered  at 
Columbia  University  in  the  winter  of  1892-93,  in  which  I  endeavoured 
to  give  to  an  audience  of  general  university  students  some  account 
of  recent  advances  in  cellular  biology,  and  more  especially  to  trace 
the  steps  by  which  the  problems  of  evolution  have  been  reduced  to 
problems  of  the  cell.  It  was  my  first  intention  to  publish  these 
lectures  in  a  simple  and  general  form,  in  the  hope  of  showing  to 
wider  circles  how  the  varied  and  apparently  heterogeneous  cell- 
researches  of  the  past  twenty  years  have  grown  together  in  a 
coherent  group,  at  the  heart  of  which  are  a  few  elementary  phe- 
nomena, and  how  these  phenomena,  easily  intelligible  even  to  those 
having  no  special  knowledge  of  the  subject,  are  related  to  the 
problems  of  development.  Such  a  treatment  was  facilitated  by 
the  appearance,  in  1893,  of  Oscar  Hertwig's  invaluable  book  on 
the  cell,  which  brought  together,  in  a  form  well  designed  for  the 
use  of  special  students,  many  of  the  more  important  results  of 
modern  cell-research.  I  am  glad  to  acknowledge  my  debt  to  Hert- 
wig's book  ;  but  it  is  proper  to  state  that  the  present  volume  was 
fully  sketched  in  its  main  outlines  at  the  time  the  Zelle  tind  Gewebe 
appeared.  Its  completion  was,  however,  long  delayed  by  investiga- 
tions which  I  undertook  in  order  to  re-examine  the  history  of  the 
centrosomes  in  the  fertilization  of  the  ^g^,  —  a  subject  which  had 
been  thrown  into  such  confusion  by  Fol's  extraordinary  account  of 
the  "  Quadrille  of  Centres  "  in  echinoderms  that  it  seemed  for  a  time 
impossible  to  form  any  definite  conception  of  the  cell  in  its  relation 
to  inheritance.  By  a  fortunate  coincidence  the  same  task  was  inde- 
pendently undertaken,  nearly  at  the  same  time,  by  several  other 
investigators.  The  concordant  results  of  these  researches  led  to  a 
decisive  overthrow  of  Fol's  conclusions,  and  the  way  was  thus  cleared 
for  a  return  to  the  earlier  and  juster  views  founded  by  Hertwig, 
Strasburger,  and  Van  Beneden,  and  so  lucidly  and  forcibly  developed 
by  Boveri. 

The  rapid  advance  of  discovery  in  the  mean  time  has  made  it 
seem  desirable  to  amplify  the  original  plan  of  the  work,  in  order  to 
render  it  useful  to  students  as  well  as  to  more  general  readers ;  and 
to  this  end  it  has  been  found  necessary  to  go  over  a  considerable 


Vlll  PREFACE 

part  of  the  ground  already  so  well  covered  by  Hertwig.^  This  book 
does  not,  however,  in  any  manner  aim  to  be  a  treatise  on  general 
histology,  or  to  give  an  exhaustive  account  of  the  cell.  It  has  rather 
been  my  endeavour  to  consider,  within  moderate  limits,  those  features 
of  the  cell  that  seem  more  important  and  suggestive  to  the  student 
of  development,  and  in  some  measure  to  trace  the  steps  by  which  our 
present  knowledge  has  been  acquired.  A  work  thus  limxited  neces- 
sarily shows  many  gaps  ;  and  some  of  these,  especially  on  the  botani- 
cal side,  are,  I  fear,  but  too  obvious.  On  its  historical  side,  too,  the 
subject  could  be  traced  only  in  its  main  outlines,  and  to  many 
investigators  of  whose  results  I  have  made  use  it  has  been  impossible 
to  do  full  justice. 

To  the  purely  speculative  side  of  the  subject  I  do  not  desire  to 
add  more  than  is  necessary  to  define  some  of  the  problems  still  to  be 
solved ;  for  I  am  mindful  of  Blumenbach's  remark  that  while  Drelin- 
court  rejected  two  hundred  and  sixty-two  "groundless  hypotheses" 
of  development,  "  nothing  is  more  certain  than  that  Drelincourt's 
own  theory  formed  the  two  hundred  and  sixty-third."  ^  I  have  no 
wish  to  add  another  to  this  list.  And  yet,  even  in  a  field  where 
standpoints  are  so  rapidly  shifting  and  existing  views  are  still  so 
widely  opposed,  the  conclusions  of  the  individual  observer  may  have 
a  certain  value  if  they  point  the  way  to  further  investigation  of  the 
facts.  In  this  spirit  I  have  endeavoured  to  examine  some  of  the  more 
important  existing  views,  to  trace  them  to  their  sources,  and  in  some 
measure  to  give  a  critical  estimate  of  their  present  standing,  in  the 
hope  of  finding  suggestion  for  further  research. 

Every  writer  on  the  cell  must  find  himself  under  a  heavy  obliga- 
tion to  the  works  of  Van  Beneden,  Oscar  Hertwig,  Flemming,  Stras- 
burger,  and  Boveri ;  and  to  the  last-named  author  I  have  a  special 
sense  of  gratitude.  I  am  much  indebted  to  my  former  student, 
Mr.  A.  P.  Mathews,  for  calling  my  attention  to  the  importance  of 
the  recent  work  of  physiological  chemists  in  its  bearing  on  the 
problems  of  synthetic  metabolism.  The  views  developed  in  Chap- 
ter VII.  have  been  considerably  influenced  by  his  suggestions,  and 
this  subject  will  be  more  fully  treated  by  him  in  a  forthcoming  work ; 
but  I  have  endeavoured  as  far  as  possible  to  avoid  anticipating  his  own 
special  conclusions.  Among  many  others  to  whom  I  am  indebted 
for  kindly  suggestion  and  advice,  I  must  particularly  mention  my 
ever  helpful  friend.  Professor  Henry  F.  Osborn,  and  Professors 
J.  E.  Humphrey,  T.  H.  Morgan,  and  F.  S.  Lee. 

In  copying  so  great  a  number  of  figures  from  the  papers  of  other 

^  Henneguy's  Le(ons  stir  la  cellule  is  received,  too  late  for  further  notice,  as  this  volume 
is  going  through  the  press. 
2  Allen  Thomson. 


PREFACE  ix 

iivestigators,  I  must  make  a  virtue  of  necessity.  Many  of  the  facts 
could  not  possibly  have  been  illustrated  by  new  figures  equal  in  value 
to  those  of  special  workers  in  the  various  branches  of  cytological 
research,  eA^en  had  the  necessary  material  and  time  been  available. 
But,  apart  from  this,  modern  cytology  extends  over  so  much  debatable 
ground  that  no  general  work  of  permanent  value  can  be  written  that 
does  not  aim  at  an  objective  historical  treatment  of  the  subject;  and 
I  believe  that  to  this  end  the  results  of  investigators  should  as  far  as 
practicable  be  set  forth  by  means  of  their  original  figures.  Those 
for  which  no  acknowledgment  is  made  are  original  or  taken  from 
my  own  earlier  papers. 

The  arrangement  of  the  literature  lists  is  as  follows.  A  general 
list  of  all  the  works  referred  to  in  the  text  is  given  at  the  end  of  the 
book  (p.  343).  These  are  arranged  in  alphabetical  order,  and  are 
referred  to  in  the  text  by  name  and  date,  according  to  Mark's  con- 
venient system.  In  order,  however,  to  indicate  to  students  the  more 
important  references  and  partially  to  classify  them,  a  short  separate 
list  is  given  at  the  end  of  each  chapter.  The  chapter-lists  include 
only  a  few  selections  from  the  general  list,  comprising  especially 
works  of  a  general  character  and  those  in  which  reviews  of  the 
special  literature  may  be  found. 

E.  B.  W. 

Columbia  University,  New  York, 
July,  1S96. 


TABLE    OF   CONTENTS 


INTRODUCTION 

PAGE 

I.isT  OF  Figures         .        .        .        .      " xv 

Historical  Sketch  of  the  Cell-theory;  its  Relation  to  the  Evolution-theory.  Earlier 
Views  of  Inheritance  and  Development.  Discovery  of  the  Germ-cells.  Cell- 
division,  Cleavage,  and  Development.    Modern  Theories  of  Inheritance.    Lamarck, 

Darwin,  and  Weismann i 

Literature 12 

CHAPTER   I 

General  Sketch  of  the  Cell 

A.  General  Morphology  of  the  Cell 14 

B.  Structural  Basis  of  Protoplasm .17 

C.  The  Nucleus 22 

1.  General  Structure 23 

2.  Finer  Structure  of  the  Nucleus    .........  27 

3.  Chemistry  of  the  Nucleus    ..........  28 

D.  The  Cytoplasm 29 

E.  The  Centrosome     . 3^ 

F.  Other  Cell-organs -37 

G.  The  Cell-membrane 3^ 

H.    Polarity  of  the  Cell 38 

I.      The  Cell  in  Relation  to  the  Multicellular  Body 41 

Literature,  I.         .... 43 

CHAPTER   II 
Cell-Division 

A.  Outline  of  Indirect  Division  or  Mitosis  . 47 

B.  Origin  of  the  Mitotic  Figure 53 

C.  Modifications  of  Mitosis         .         .         .  .         .         .         .         •         •  •         -57 

1.  Varieties  of  the  Mitotic  Figure •         -57 

2.  Heterotypical  Mitosis           . 60 

3.  Bivalent  and  Plurivalent  Chromosomes 61 

4.  Mitosis  in  the  Unicellular  Plants  and  Animals 62 

5.  Pathological  Mitoses 67 

I).     The  Mechanism  of  Mitosis 7° 

1.  Inunction  of  the  Amphiaster         . 7° 

(a)  Theory  of  Fibrillar  Contractility 7^ 

(/>)   Other  Theories 75 

2.  Division  of  the  Chromosomes       ....•••••  77 

xi 


XU  TABLE    OF   CONTENTS 

I'AGK 

E.  Direct  or  Amitotic  Division 80 

1.  General  Sketch 80 

2.  Centrosome  and  Attraction-sphere  in  Amitosis    .         .         .         .  .81 

3.  Biological  Significance  of  Amitosis 82 

F.  Summary  and  Conclusion 85 

Literature,  II 80 

CHAPTER   HI 

The  Germ-Cells 

A.  The  Ovum 90 

1.  The  Nucleus 92 

2.  The  Cytoplasm 94 

3.  The  Egg-envelopes .96 

B.  The  SpermatozoSn .       98 

1.  The  Flagellate  Spermatozoon       .........       99 

2.  Other  Forms  of  Spermatozoa .         .         .106 

3.  Paternal  Germ-cells  of  Plants 106 

C.  Origin  and  Growth  of  the  Germ-cells 108 

D.  Growth  and  Differentiation  of  the  Germ-cells 113 

1.  The  Ovum 113 

{a)  Growth  and  Nutrition    .         .         .         .         .         .         .  ■     ^3 

ib)  Differentiation  of  the  Cytoplasm.     Deposit  of  Deutoplasm     .         .     115 
(<:)  Yolk-nucleus 118 

2.  Formation  of  the  Spermatozoon .122 

E.  Staining-reactions  of  the  Germ-nuclei 127 

Literature,  III. 128 

CHAPTER   IV 
Fertilization  of  the  Ovum 

A.  General  Sketch 130 

1.  The  Germ-nuclei  in  Fertilization  .         .         .         .         .         .         .         .132 

2.  The  Centrosome  in  Fertilization -135 

B.  Union  of  the  Germ-cells         ...........     145 

1.  Immediate  Results  of  Union 149 

2.  Paths  of  the  Germ-nuclei .         .         -151 

3.  Union  of  the  Germ-nuclei.     The  Chromosomes   ......     153 

C.  Centrosome  and  Archoplasm  in  Fertilization 156 

D.  Fertilization  in  Plants    .         .         .         .         .         .         .         ,         .         .         .         .160 

E.  Conjugation  in  Unicellular  Forms 163 

F.  Summary  and  Conclusion .         .         .170 

Literature,  IV .         .         .         .         .         -171 

CHAPTER   V 
Reduction  of  the  Chromosomes,  Oogenesis  and  Spermatogenesis 

A.  General  Outline      ,         .         . 174 

1.  Reduction  in  the  Female.     The  Polar  Bodies      .         .         .         .         .         -175 

2.  Reduction  in  the  Male.     Spermatogenesis  .         .         .         .         .         .         .180 

3.  Theoretical  Significance  of  Maturation .182 

B.  Origin  of  the  Tetrads 186 

1.  General  Sketch 186 

2.  Detailed  Evidence       . 187 

C.  The  Early  History  of  the  Germ-nuclei    .........  193 

D.  Reduction  in  the  Plants 195 


TABLE    OF   CONTENTS  xiii 

PAGE 

E.  Reduction  in  Unicellular  Forms loj^ 

F.  Divergent  Accounts  of  Reduction ion 

I.  Formation  of  Tetrads  by  Conjugation          .......  loo 

G.  Maturation  of  Parthenogenetic  Eggs 202 

II.    Summary  and  Conclusion       ••........,  201: 

Appendix 208 

1 .  Accessory  Cells  of  the  Testis 208 

2.  A  mitosis  in  the  Early  Sex-cells 209 

Literature,  Y. 209 

CHAPTER  VI 

Some  Problems  of  Cell-Organization 

A.  The  Nature  of  Cell-organs      .         .         .         , 211 

B.  Structural  Basis  of  the  Cell     .          .       - 212 

I.  Nucleus  and  Cytoplasm 214 

C.  Morphological  Composition  of  the  Nucleus 215 

I.  The  Chromatin    . 215 

{a)   Hypothesis  of  the  Individuality  of  the  Chromosomes     .         .         .215 

(/^)  Composition  of  the  Chromosomes 221 

D.  Chromatin,  Linin,  and  the  Cytoreticulum        .         .         .         .         .         .         .         .  223 

E.  The  Centrosome     .............  224 

F.  The  i\rchoplasmic  Structures          . 229 

1.  Asters  and  Spindle'     ...........  229 

2.  The  Attraction-sphere 232 

G.  Summary  and  Conclusion       .         .         .         .         . 236 

Literature,  VI. 237 

CHAPTER  VII 

Some  Aspects  of  Cell-Chemistry  and  Cell-Physiology 

A.  Chemical  Relations  of  Nucleus  and  Cytoplasm      .         .  - 238 

1.  The  Proteids  and  their  Allies 239 

2.  The  Nuclein  Series 240 

3.  Staining-reactions  of  the  Nuclein  Series 242 

B.  Physiological  Relations  of  Nucleus  and  Cytoplasm         ......  248 

1.  Experiments  on  Unicellular  Organisms 248 

2.  Position  and  Movements  of  the  Nucleus 252 

3.  The  Nucleus  in  Mitosis 256 

4.  The  Nucleus  in  Fertilization 257 

5.  The  Nucleus  in  Maturation 259 

C.  The  Centrosome 259 

D.  Summary  and  Conclusion        ...........  261 

Literature,  VII. 263 

CHAPTER  VIII 
Cell-Division  and  Development 


A.  Geometrical  Relations  of  Cleavage-forms 

B.  Promorphological  Relations  of  Cleavage 

1.  Promorphology  of  the  Ovum 

{a)   Polarity  and  the  Egg-axis 

(J))   Axial  Relations  of  the  Primary  Cleavage- 

{c)  Other  Promorphological  Characters  of  the  Ovum 

2.  Meaning  of  the  Promorphology  of  the  Ovum 


planes 


265 
278 
278 
278 
280 
282 
285 


XIV  TABLE    OF  CONTENTS 

PAGE 

C.  The  Energy  of  Division         ...                            .....  280 

D.  Cell-division  and  Growth        ....                            .....  20^ 

Literature,  VIII .         ,         !         .         !  294 

CHAFFER  IX 

Theories  of  Inheritance  and  Development 

A.     The  Theory  of  Germinal  Localization    .........  296 

li.     The  Idioplasm  Theory ^qq 

C.  Union  of  the  Two  Theories ^02 

D.  The  Roux-Weismann  Theory  of  Development .  303 

E.  Critique  of  the  Roux-Weismann  Theory 306 

F.  On  the  Nature  and  Causes  of  Differentiation 311 

G.  The  Nucleus  in  Later  Development 321 

H.    The  External  Conditions  of  Development t.t.'^ 

I.      Development,  Inheritance,  and  Metabolism 326 

J.      Preformation  and  Epigenesis.     The  Unknown  Factor  in  Development  .         .         -327 

Literature,  IX , ,,. 

•  jj" 

Glossary -.^^ 

General  Literature-List 34^ 

Index  of  Authors ^^9 

Index  of  Subjects ^5^ 


LIST   OF    FIGURES 


PAGE 

Epidermis  of  larval  salamander 2 

Amasba  Proteus 4 

Cleavage  of  the  ovum  in  Toxopneustes  8 

Diagram  of  inheritance. 11 

Diagram  of  a  cell 14 

Spermatogonium  of  salamander 15 

Group  of  cells,  showing  cytoplasm,  nu- 
cleus, and  centrosome 16 

Alveolar  or   foam-structure   of  proto- 
plasm, according  to  Biitschli 18 

Living   cells   of  salamander,  showing 

fibrillar  structure 20 

Nuclei  from  the  crypts  of  Lieberkiihn.  24 

Special  forms  of  nuclei 25 

Diffused  nucleus  in  Trachelocerca. ...  26 

Ciliated  cells 30 

Nephridial  cell  of  Clepsine 32 

Nerve-cell  of  frog 33 

Diagram  of  dividing  cell 35 

Diagrams  of  cell-polarity 39 

Remak's  scheme  of  cell-division 46 

Diagram  of  the  prophases  of  mitosis.  .  48 

Diagram  of  later  phases  of  mitosis.  ...  50 

Prophases  in  salamander  cells 54 

Metaphase  and  anaphases  in  salaman- 
der cells 55 

Telophases  in  salamander  cells 56 

Middle  phases  of  mitosis  in  Ascaris- 

eggs 58 

Mitosis  in  pollen-mother-cells  of  lily.  .  59 

Heterotypical  mitosis 60 

Mitosis  in  Infusoria 62 

Mitosis  in  Euglypha 63 

Mitosis  in  Euglena 64 

Mitosis  in  Noctlluca 65 

Mitosis  in  Act'mosphcBrluni 66 

Patliological  mitoses  in  cancer-cells..  .  68 

Pathological  mitosis  caused  by  poisons  6g 

Mechanism  of  mitosis  in  Ascaris 71 

Leucocytes 72 

Pigment-cells 73 

Mitosis  in  the  egg  of  Toxopneustes.  ...  76 

Nuclei  in  the  spireme-stage 78 

Early  division  of  chromatin  in  Ascaris  79 

Amitotic  division 81 

Volvox 89 

Ovum  of  Toxopneustes 91 


PAGE 

Ovum  of  Nereis 95 

Insect-egg 96 

Micropyle  in  Argonauta 97 

Germ-cells  of  Volvox 98 

Diagram  of  the  flagellate  spermatozoon  99 
Spermatozoa  of  fishes  and  amphibia. .  100 
Spermatozoa  of  birds  and  other  ani- 
mals   102 

Spermatozoa  of  mammals 104 

Unusual  forms  of  spermatozoa 105 

Spermatozoids  of  Chara 106 

Spermatozoids  of  various  plants 107 

Germ- cells  of  Hydractinia.. . . : 109 

Primordial  germ-cells  of  Ascaris no 

Primordial  germ-cells  of  Cyclops 112 

Egg  and  nurse-cell  in  Ophryotrocha.  .  .  114 

Ovarian  eggs  of  insects 115 

Young  ovarian  eggs  of  various  animals  116 
Young  ovarian  eggs  of  birds  and  mam- 
mals    118 

Young  ovarian  eggs  of  earthworm. ...  120 

Formation  of  the  spermatozoon 124 

Transformation    of  the  spermatids  of 

the  salamander 125 

Fertilization  of  Physa 131 

Fertilization  of  Ascaris 133 

Germ-nuclei  of  nematodes 134 

Fertilization  of  the  mouse 136 

Fertilization  of  Pterotrachea 137 

Entrance  and  rotation  of  sperm-head 

in   Toxopneustes 138 

Conjugation  of  the  germ-nuclei  in  Tox- 

optieustes 139 

Fertilization  of  Nereis 141 

Fertilization  of  Cyclops 142 

Continuity  of  centrosomes  in    'Jhalas- 

sema 144 

Entrance  of  spermatozoon  into  the  egg  146 

Pathological  polyspermy 147 

Polar  rings  of  Clepsine 150 

Paths    of    the    germ-nuclei     in    Toxo- 
pneustes    152 

Fertilization  of  Myzostoma 158 

Fertilization  of  Pilularia 160 

Fertilization  of  the  lily 161 

Diagram  of  conjugation  in  Infusoria.  .  164 

Conjugation  of  Paramcecium 166 


XVI 


LIST   Of  FIGURES 


PAGE    ; 

83.  Conjugation  of  Vorticella 167  112. 

84.  Conjugation  of  Soctiluca 168  113. 

85.  Conjugation  of  Spirogyra 169 

86.  Polar  bodies  in  Toxopneustes 174    j    114. 

87.  Genesis  of  the  egg 175    |    115. 

88.  Diagram  of  formation  of  polar  bodies  177    I    116. 

89.  Polar  bodies  in  A  scar  is 178 

90.  Genesis  of  the  spermatozoon 180  117. 

91.  Diagram  of  reduction  in  the  male.. . .   181  118. 

92.  Spermatogenesis  of  Ascaris 184 

93.  Tetrads  of  Gtyllotalpa 188  IT9. 

94.  Tetrads  and  polar  bodies  in  Cyclops..    189 

95.  Diagrams  of  tetrad-formation  in  ar-  120. 

thropods 191  121, 

96.  Germinal  vesicles  and  tetrads 192  122. 

97.  Ovary  of  Canthocamptus 194  123. 

98.  Possible  tetrad-formation  in  the  lily.  .    197  124. 

99.  Conjugation  and  reduction  in  Closte-  125. 

rium 198  126. 

100.   First  type  of  parthenogenetic  matura-  127. 

tion  in  Artemia 203  128. 

loi.  Second  type  of  parthenogenetic  mat-  129. 

uration  in  Artemia 204  130. 

102.  Modes  of  tetrad-formation  contrasted  206  131. 

103.  Abnormalities  in   the   fertilization  of  132. 

Ascaris 216  133. 

104.  Individuality  of  chromosomes  in  As- 

caris    217  134. 

105.  Independence  of  chromosomes  in  fer- 

tilization of  Cyclops 218  135. 

106.  Hybrid  fertilization  of  Ascaris 220 

107.  Mitosis  with  intra-nuclear  centrosome  136. 

in  Ascaris 225 

108.  Diagram  of  different  types  of  centro-  137. 

some  and  centrosphere 233 

109.  Structure  of  the  aster  in  spermatogo-  138. 

nium  of  salamander 234  139. 

no.   History  of  chromosomes  in  the  germi-  140. 

nal  vesicle  of  sharks 245  141. 

III.   Nucleated  and  enucleated  fragments 

of  Stylonychia 249  142. 


PAGE 

Regeneration  in  Stentor 250 

Nucleated  and  enucleated  fragments 

of  Aniceha 25 1 

Position  of  nuclei  in  plant-cells 253 

Ovary  of  Forjicula 255 

Normal    and    dwarf    larvae    of    sea- 
urchins  258 

Supernumerary  centrosome  in  Ascaris  260 
Cleavage  of  dispermic  egg  of   Toxo- 
pneustes     261 

Geometrical    relations    of    cleavage- 
planes  in  plants 266 

Cleavage  of  Syvapta 268 

Cleavage  of  Polygordius 269 

Cleavage  of  Nereis 271 

Variations  in  the  third  cleavage 272 

Meroblastic  cleavage  in  the  squid 273 

Teloblasts  of  the  earthworm 274 

Bilateral  cleavage  in  tunicates 281 

Bilateral  cleavage  in  Loligo 282 

Eggs  of  Loligo 283 

Eggs  and  embryos  of  Corixa 284 

Variations  in  axial  relations  of  Cyclops  286 

Half-embryos  of  the  frog 299 

Half  and  whole  cleavage  in  sea-urchins  306 
Normal  and  dwarf  gastrulas  of  Amphi- 

oxus 307 

Dwarf  and  double  embryos  of  Amphi- 

oxus 308 

Cleavage   of  sea-urchin   eggs   under 

pressure 309 

Cleavage  of  A^^r^ii-eggs  under  press- 
ure     310 

Diagrams  of  cleavage  in  annelids  and 

polyclades 313 

Partial  larvae  of  ctenophores 314 

Partial  cleavage  in  Ilyatiassa 316 

Double  embryos  of  frog 318 

Normal  and  modified  larvae  of  sea- 
urchins 324 

Regeneration  in  ccelenterates 325 


INTRODUCTION 


o>^< 


"/edes  Thier  erscheint  ah  eine  Sumine  vitaler  Einheiten,  von  denen  jede  den  vollen 
Charakter  des  Lebens  an  sich  tragt."       ^  VlRCHOW.^ 

During  the  half-century  that  has  elapsed  since  the  enunciation  of 
the  cell-theory  by  Schleiden  and  Schwann,  in  1838-39,  it  has  become 
ever  more  clearly  apparent  that  the  key  to  all  ultimate  biological 
problems  must,  in  the  last  analysis,  be  sought  in  the  cell.  It  was  the 
cell-theory  that  first  brought  the  structure  of  plants  and  animals  under 
one  point  of  view  by  revealing  their  common  plan  of  organization.  It 
was  through  the  cell-theory  that  Kolliker  and  Remak  opened  the  way 
to  an  understanding  of  the  nature  of  embryological  development,  and 
the  law  of  genetic  continuity  lying  at  the  basis  of  inheritance.  It 
was  the  cell-theory  again  which,  in  the  hands  of  Virchow  and  Max 
Schultze,  inaugurated  a  new  era  in  the  history  of  physiology  and 
pathology,  by  showing  that  all  the  various  functions  of  the  body,  in 
health  and  in  disease,  are  but  the  outward  expression  of  cell-activi- 
ties. And  at  a  still  later  day  it  was  through  the  cell-theory  that  Hert- 
wig,  Fol,  Van  Beneden,  and  Strasburger  solved  the  long-standing 
riddle  of  the  fertilization  of  the  ^^^^  and  the  mechanism  of  hereditary 
transmission.  No  other  biological  generalization,  save  only  the  theory 
of  organic  evolution,  has  brought  so  many  apparently  diverse  phe- 
nomena under  a  common  point  of  view  or  has  accomplished  more 
for  the  unification  of  knowledge.  The  cell-theory  must  therefore  be 
placed  beside  the  evolution-theory  as  one  of  the  foundation  stones  of 
modern  biology. 

And  yet  the  historian  of  latter-day  biology  cannot  fail  to  be  struck 
with  the  fact  that  these  two  great  generalizations,  nearly  related  as 
they  are,  have  been  developed  along  widely  different  lines  of  research, 
and  have  only  within  a  very  recent  period  met  upon  a  common  ground. 
The  theory  of  evolution  originally  grew  out  of  the  study  of  natural 
history,  and  it  took  definite  shape  long  before  the  ultimate  structure 
of  living  bodies  was  in  any  degree  comprehended.     The  evolutionists 

1  Cellular pathologie,  p.  12,  1858. 
B  I 


2  INTRODUCTION 

of  the  Lamarckian  period  gave  little  heed  to  the  finer  details  of 
internal  organization.  They  were  concerned  mainly  with  the  more 
obvious  characters  of  plants  and  animals  —  their  forms,  colours, 
habits,  distribution,  their  anatomy  and  embryonic  development  — 
and  with  the  systems  of  classification  based  upon  such  characters ; 
and  long  afterwards  it  was,  in  the  main,  the  study  of  like  characters 
with  reference  to  their  historical  origin  that  led  Darwin  to  his  splen- 

a 


X  b 

Fig.  I. — A  portion  of  the  epidermis  of  a  larval  salamander  {Amblystoma)  as  seen  in  slightly 
oblique  horizontal  section,  enlarged  550  diameters.  Most  of  the  cells  are  polygonal  in  form,  con- 
tain large  nuclei,  and  are  connected  hy  delicate  protoplasmic  bridges.  Above  :ir  is  a  branched, 
dark  pigment-cell  that  has  crept  up  from  the  deeper  layers  and  lies  between  the  epidermal  cells. 
Three  of  the  latter  are  undergoing  division,  the  earliest  stage  {spireme)  at  a,  a  later  stage  (mitotic 
figure  in  the  anaphase)  at  (J,  showing  the  chromosomes,  and  a  final  stage  {telophase),  showing 
fission  of  the  cell-body,  to  the  right. 

did  triumphs.  The  study  of  microscopical  anatomy,  on  which  the 
cell-theory  was  based,  lay  in  a  different  field.  It  was  begun  and  long 
carried  forward  with  no  thought  of  its  bearing  on  the  origin  of  living 
forms ;  and  even  at  the  present  day  the  fundamental  problems  of 
organization,  with  which  the  cell-theory  deals,  are  far  less  accessible 
to  historical  inquiry  than  those  suggested  by  the  more  obvious 
external  characters  of  plants  and  animals.     Only  within  a  few  years. 


INTR  OD  UC  TION  3 

indeed,  has  the  ground  been  cleared  for  that  close  alliance  of  the 
evolutionists  and  the  cytologists  which  forms  so  striking  a  feature 
of  contemporary  biology.  We  may  best  examine  the  steps  by  which 
this  alliafhce  has  been  effected  by  an  outline  of  the  cell-theory,  fol- 
lowed by  a  brief  statement  of  its  historical  connection  with  the  evolu- 
tion-theory. 

During  the  past  thirty  years,  the  theory  of  organic  descent  has 
been  shown,  by  an  overwhelming  mass  of  evidence,  to  be  the  only 
tenable  conception  of  the  origin  of  diverse  living  forms,  however  we 
may  conceive  the  causes  of  the  process.  While  the  study  of  general 
zoology  and  botany  has  systematically  set  forth  the  results,  and  in  a 
measure  the  method,  of  organic  evolution,  the  study  of  microscopical 
anatomy  has  shown  us  the  nature  of  the  material  on  which  it  has 
operated,  demonstrating  that  the  obvious  characters  of  plants  and 
animals  are  but  varying  expressions  of  a  subtle  interior  organization 
common  to  all.  In  its  broader  outlines  the  nature  of  this  organiza- 
tion is  now  accurately  determined ;  and  the  "  cell-theory,"  by  which 
it  is  formulated,  is,  therefore,  no  longer  of  an  inferential  or  hypo- 
thetical character,  but  a  generalized  statement  of  observed  fact  which 
may  be  outlined  as  follows :  — 

In  all  the  higher  forms  of  life,  whether  plants  or  animals,  the 
body  may  be  resolved  into  a  vast  host  of  minute  structural  units 
known  as  cells,  out  of  which,  directly  or  indirectly,  every  part  is 
built  (Fig.  i).  The  substance  of  the  skin,  of  the  brain,  of  the  blood, 
of  the  bones  or  muscles  or  any  other  tissue,  is  not  homogeneous,  as  it 
appears  to  the  unaided  eye.  The  microscope  shows  it  to  be  an  aggre- 
gate composed  of  innumerable  minute  bodies,  as  if  it  were  a  colony 
or  congeries  of  organisms  more  elementary  than  itself.  These  elemen- 
tary bodies,  the  cells,  are  essentially  minute  masses  of  living  matter 
Q)X  protoplasm,  a  substance  characterized  by  Huxley  many  years  ago 
as  the  "physical  basis  of  life"  and  now  universally  recognized  as  the 
immediate  substratum  of  all  vital  action.  Endlessly  diversified  in  the 
details  of  their  form  and  structure,  cells  nevertheless  possess  a  charac- 
teristic type  6f  organization  common  to  them  all ;  hence,  in  a  certain 
sense,  they  may  be  regarded  as  elementary  organic  units  out  of 
which  the  body  is  compounded.  In  the  lowest  forms  of  life  the 
entire  body  consists  of  but  a  single  cell  (Fig.  2).  In  the  higher  multi- 
cellular forms  the  body  consists  of  a  multitude  of  such  cells  asso- 
ciated in  one  organic  whole.  Structurally,  therefore,  the  multicellular 
body  is  in  a  certain  sense  comparable  with  a  colony  or  aggregation  of 
the  lower  one-celled  forms. ^  From  the  physiological  point  of  view  a 
like  comparison  may  be  drawn.      In  the  one-celled  forms  all  of  the 

^  This  comparison  must  be  taken  with  some  reservation,  as  will  appear  beyond. 


4  INTRODUCTION 

vital  functions  are  performed  by  a  single  cell ;  in  the  higher  types  they 
are  distributed  by  a  physiological  division  of  labour  among  different 
groups  of  cells  specially  devoted  to  the  performance  of  specific 
functions.  The  cell  is  therefore  not  only  a  unit  of  structure,  but 
also  a  unit  of  function.  "  It  is  the  cell  to  which  the  consideration 
of  every  bodily  function  sooner  or  later  drives  us.  In  the  muscle- 
cell  lies  the  riddle  of  the  heart-beat,  or  of  muscular  contraction ;  in  the 
gland-cell  are  the  causes  of  secretion;  in  the  epithelial  cell,  in  the 
white  blood-cell,  lies  the  problem  of  the  absorption  of  food,  and 
the  secrets  of  the  mind  are  slumbering  in  the  ganglion-cell.  ...  If 
then  physiology  is  not  to  rest  content  with  the  mere  extension  of  our 


/•■'"  •"■•!,•,'•  <■  •0'V< .'  -S'  ■•.  V  '^^'O  ti.  '^•.■"•.•■/^. 


Fig.  2.  —  AmcBba  Proteus,  an  animal  consisting  of  a  single  naked  cell,  X  280.  (From  Sedgwick 
and  Wilson's  Biology.) 

n.  The  nucleus ;  w.v.  Water-vacuoles ;  c.v.  Contractile  vacuole ;  f.v.  Food-vacuole. 

knowledge  regarding  the  more  obvious  operations  of  the  human 
body,  if  it  would  seek  a  real  explanation  of  the  fundamental  phe- 
nomena of  life,  it  can  only  attain  its  end  through  the  study  of  cell- 
physiology  ^  ^ 

Great  as  was  the  impulse  which  the  cell-theory  gave  to  anatomical 
and  physiological  investigation,  it  did  not  for  many  years  measurably 
affect  the  more  speculative  side  of  biological  inquiry.  The  Origin  of 
Species,  published  in  1859,  scarcely  mentions  it;  nor,  if  we  except 
the  theory  of  pangenesis,  did  Darwin  attempt  at  any  later  period  to 
bring  it  into  any  very  definite  relation  to  his  views.  The  cell-theory 
first  came  in  contact  with  the  evolution-theory  nearly  twenty  years 

1  Verworn,  AUgemeine  Physiologie,  p.  53,  1895. 


INTRODUCTION  5 

later  through  researches  on  the  early  history  of  the  germ-cells  and  the 
fertilization  of  the  ovum.  Begun  in  1873-74  by  Auerbach,  Fol,  and 
Biitschli,  and  eagerly  followed  up  by  Oscar  Hertwig,  Van  Beneden, 
Strasburgfer,  and  a  host  of  later  workers,  these  investigations  raised 
wholly  new  questions  regarding  the  mechanism  of  development  and 
the  role  of  the  cell  in  hereditary  transmission.  The  identification  of 
the  cell-tmcleiis  as  the  vehicle  of  inheritance,  made  independently  and 
almost  simultaneously  in  1884-85  by  Oscar  Hertwig,  Strasburger, 
Kolliker,  and  Weismann,  must  be  recognized  as  the  first  definite 
advance  ^  towards  the  internal  problems  of  inheritance  through  the 
cell-theory ;  and  the  discussions  to  which  it  gave  rise,  in  which  Weis- 
mann has  taken  the  foremost  place,  must  be  reckoned  as  the  most 
interesting  and  significant  of  the  post-Darwinian  period. 

These  discussions  have  set  forth  in  strong  relief  the  truth  that 
the  general  problems  of  evolution  and  heredity  are  indissolubly 
bound  up  with  those  of  cell-structure  and  cell-action.  This  can  best 
be  appreciated  from  an  historical  point  of  view.  The  views  of  the 
early  embryologists  in  regard  to  inheritance  were  vitiated  by  their 
acceptance  of  the  Greek  doctrine  of  the  equivocal  or  spontaneous 
generation  of  life ;  and  even  Harvey  did  not  escape  this  pitfall,  near 
as  he  came  to  the  modern  point  of  view.  "The  ^ZZ^''  ^^  says,  "is 
the  mid-passage  or  transition  stage  between  parents  and  offspring, 
between  those  who  are,  or  were,  and  those  who  are  about  to  be  ; 
it  is  the  hinge  or  pivot  upon  which  the  whole  generation  of  the 
bird  revolves.  The  ^^^  is  the  terminus  from  which  all  fowls,  male 
and  female,  have  sprung,  and  to  which  all  their  lives  tend  —  it  is  the 
result  which  nature  has  proposed  to  herself  in  their  being.  And 
thus  it  comes  that  individuals  in  procreating  their  like  for  the  sake 
of  their  species,  endure  forever.  The  ^gg,  I  say,  is  a  period  or  por- 
tion of  this  eternity."  2 

This  passage  appears  at  first  sight  to  be  a  close  approximation  to 
the  modern  doctrine  of  germinal  continuity  about  which  all  theories 
of  heredity  are  revolving.  To  the  modern  student  the  germ  is,  in 
Huxley's  words,  simply  a  detached  living  portion  of  the  substance 
of  a  pre-existing  living  body^  carrying  with  it  a  definite  structural 
organization  characteristic  of  the  species.  Harvey's  view  is  only 
superficially  similar  to  this;  for,  as  Huxley  pointed  out,  it  was  obscured 
by  his  behef  that  the  germ  might  arise  "spontaneously,"  or  through 

^  It  must  not  be  forgotten  that  Haeckel  expressed  the  same  view  in  1866 — only,  how- 
ever, as  a  speculation,  since  the  data  necessary  to  an  inductive  conclusion  were  not  obtained 
until  long  afterwards.  "The  internal  nucleus  provides  for  the  transmission  of  hereditary 
characters,  the  external  plasma  on  the  other  hand  for  accommodation  or  adaptation  to  the 
external  world"  (^Geti.  Morph.,  p.  287-9). 

-  De  Generatione,  1 651;   Trans.,  p.  271. 

'^  Evolution  in  Biology,  1878;    Science  a }id  Culture,  p.  291. 


6  JNTMODUCTION 

the  influence  of  a  mysterious  "  calidum  innatum,''  out  of  not-living 
matter.  Whitman,  too,  in  a  recent  brilliant  essay,^  has  shown  how 
far  Harvey  was  from  any  real  grasp  of  the  law  of  genetic  continuity, 
which  is  well  characterized  as  the  central  fact  of  modern  biology. 
Neither  could  the  great  physiologist  of.  the  seventeenth  century  have 
had  the  remotest  conception  of  the  actual  structure  of  the  (:t^^^.  The 
cellular  structure  of  living  things  was  not  comprehended  until  nearly 
two  centuries  later.  The  spermatozoon  was  still  undiscovered,  and  the 
nature  of  fertilization  was  a  subject  of  fantastic  and  baseless  specu- 
lation. For  a  hundred  years  after  Harvey's  time  embryologists 
sought  in  vain  to  penetrate  the  mysteries  enveloping  the  beginning 
of  the  individual  life,  and  despite  their  failure  the  controversial  writ- 
ings of  this  period  form  one  of  the  most  interesting  chapters  in  the 
history  of  biology.  By  the  extreme  "evolutionists"  or  "  praef  orma- 
tionists"  the  ^gg  was  believed  to  contain  an  embryo  fully  formed  in 
miniature,  as  the  bud  contains  the  flower  or  the  chrysalis  the  butter- 
fly. Development  was  to  them  merely  the  unfolding  of  that  which 
already  existed ;  inheritance,  the  handing  down  from  parent  to  child 
of  an  infinitesimal  reproduction  of  its  own  body.  It  was  the  service 
of  Bonnet  to  push  this  conception  to  its  logical  consequence,  the 
theory  of  ernbottement  or  encasement,  and  thus  to  demonstrate  the 
absurdity  of  its  grosser  forms ;  for  if  the  o^^^^  contains  a  complete 
embryo,  this  must  itself  contain  eggs  for  the  next  generation,  these 
other  eggs  in  their  turn,  and  so  ad  infinitum^  like  an  infinite  series 
of  boxes,  one  within  another  —  hence  the  term  "  emboitement." 
Bonnet  himself  renounced  this  doctrine  in  his  later  writings,  and 
Caspar  Frederich  Wolff  (1759)  led  the  way  in  a  return  to  the  teach- 
ings of  Harvey,  showing  by  precise  actual  observation  that  the  ^g'g 
does  not  at  first  contain  any  formed  embryo  whatever  ;  that  the  struct- 
ure is  wholly  different  from  that  of  the  adult ;  that  development  is  not 
a  mere  process  of  unfolding,  but  a  progressive  process,  involving  the 
continual  formation,  one  after  another,  of  new  parts,  previously  non- 
existent, as  such.  This  is  somewhat  as  Harvey,  himself  following 
Aristotle,  had  conceived  it  —  a  process  of  epigenesis  as  opposed  to 
evolutioji.  Later  researches  established  this  conclusion  as  the  very 
foundation  of  embryological  science. 

But  although  the  external  nature  of  development  was  thus  deter- 
mined, the  actual  structure  of  the  egg  and  the  mechanism  of  inheri- 
tance remained  for  nearly  a  century  in  the  dark.  It  was  reserved 
for  Schwann  (1839)  and  his  immediate  followers  to  recognize  the 
fact,  conclusively  demonstrated  by  all  later  researches,  that  the  egg 
is  a  cell  having   the  same  essential  structure  as  other  cells  of   the 

1  Evolution  and  Epigenesis,  Wood's  HoU  Biological  Lectures,  1894. 


INTRODUCTION  7 

body.  And  thus  the  wonderful  truth  became  manifest  that  a  single 
cell  may  contain  within  its  microscopic  compass  the  sum-total  of 
the  heritage  of  the  species.  This  conclusion  first  reached  in  the 
case  of  the  female  sex  was  soon  afterwards  extended  to  the  male 
as  well.  **  Since  the  time  of  Leeuwenhoek  (1677)  it  had  been  known 
that  the  sperm  or  fertilizing  fluid  contained  innumerable  minute 
bodies  endowed  in  nearly  all  cases  with  the  power  of  active  move- 
ment, and  therefore  regarded  by  the  early  observers  as  parasitic 
animalcules  or  infusoria,  a  view  which  gave  rise  to  the  name  sperma- 
tozoa (sperm-animals)  by  which  they  are  still  generally  known.^  As 
long  ago  as  1786,  however,  it  was  shown  by  Spallanzani  that  the 
fertilizing  power  must  lie  in  the  spermatozoa,  not  in  the  liquid  in 
which  they  swim,  because  the  spermatic  fluid  loses  its  power  when 
filtered.  Two  years  after  the  appearance  of  Schwann's  epoch-mak- 
ing work  Kolliker  demonstrated  (1841)  that  the  spermatozoa  arise 
directly  from  cells  in  the  testis,  and  hence  cannot  be  regarded  as 
parasites,  but  are,  like  the  ovum,  derived  from  the  parent-body.  Not 
until  1865,  however,  was  the  final  proof  attained  by  Schweigger- 
Seidel  and  La  Valette  St.  George  that  the  spermatozoon  contains 
not  only  a  nucleus,  as  Kolliker  believed,  but  also  cytoplasm.  It 
was  thus  shown  to  be,  like  the  Q,gg,  a  single  cell,  peculiarly  modified 
in  structure,  it  is  true,  and  of  extraordinary  minuteness,  yet  on  the 
whole  morphologically  equivalent  to  other  cells.  A  final  step  was 
taken  ten  years  later  (1875),  when  Oscar  Hertwig  established  the 
all-important  fact  that  fertilization  of  the  ^^^  is  accomplished  by 
its  union  with  one  spermatozoon,  and  one  only.  In  sexual  repro- 
duction, therefore,  each  sex  contributes  a  single  cell  of  its  own  body 
to  the  formation  of  the  offspring,  a  fact  which  beautifully  tallies 
with  the  conclusion  of  Darwin  and  Galton  that  the  sexes  play,  on 
the  whole,  equal,  though  not  identical  parts  in  hereditary  trans- 
mission. The  ultimate  problems  of  sex,  fertilization,  inheritance, 
and  development  were  thus  shown  to  be  cell-pivblems. 

Meanwhile,  during  the  years  immediately  following  the  announce- 
ment of  the  cell-theory  the  attention  of  investigators  was  especially 
focussed  upon  the  question  :  How  do  the  cells  of  the  body  arise } 
Schwann  and  Schleiden  held  that  cells  might  arise  in  two  different 
ways;  viz.  either  by  the  division  or  fission  of  a  pre-existing  mother- 
cell,  or  by  "  free  cell-formation,"  new  cells  arising  in  the  latter  case 
not  from  pre-existing  cells,  but  by  crystallizing,  as  it  were,  out  of 
a  formative  or  nutritive  substance,  termed  the  "cytoblastema."  It 
was  only  after  many  years  of  painstaking  research  that  "  free  cell- 

1  The  discovery  of  the  spermatozoa  is  generally  accredited  to  Ludwig  Hamm,  a  pupil 
of  Leeuwenhoek  (1677),  though  Hartsoeker  afterwards  claimed  the  merit  of  having  seen 
them  as  early  as  1674  (Dr.  Allen  Thomson). 


8 


INTRODUCTION 


formation  "  was  absolutely  proved  to  be  a  myth,  though  many  of 
Schwann's  immediate  followers  threw  doubts  upon  it,  and  as  early 
as  1855  Virchow  positively  maintained  the  universality  of  cell-divis* 
ion,  contending  that  every  cell  is  the  offspring  of  a  pre-existing 
parent-cell,  and  summing  up  in  the  since  famous  aphorism,  ''  omnis 


Fig.  3.  —  Cleavage  of  the  ovum  of  the  sea-urchin  Toxopneustes,  x  330,  from  life.  The  suc- 
cessive divisions  up  to  the  i6-cell  stage  (//)  occupy  about  two  hours.  /  is  a  section  of  the  embryo 
(blastula)  of  three  hours,  consisting  of  approximately  128  cells  surrounding  a  central  cavity  or 
blastoccel. 


celhila  e  celliila''^  At  the  present  day  this  conclusion  rests  upon  a 
foundation  so  firm  that  we  are  justified  in  regarding  it  as  a  universal 
law  of  development. 

Now,  if  the  cells  of  the  body  always  arise  by  the  division  of  pre- 
existing cells,  all  must  be  traceable  back  to  the  fertilized  egg-cell  as 

1  Arch,   fiir  Path.  Anal.,  VIII.,  p.  23,  1855. 


INTR  OD  UC7'ION  9 

their  common  ancestor.  Such  is,  in  fact,  the  case  in  every  plant  and 
animal  whose  development  is  accurately  known.  The  first  step  in 
development  consists  in  the  division  of  the  ^^g  into  two  parts,  each 
of  which  is  a  cell,  like  the  Q^g%  itself.  The  two  then  divide  in  turn  to 
form  f our<  eight,  sixteen,  and  so  on  in  more  or  less  regular  progres- 
sion (Fig.  3)  until  step  by  step  the  Q,gg  has  split  up  into  the  multitude 
of  cells  which  build  the  body  of  the  embryo,  and  finally  of  the  adult. 
This  process,  known  as  the  cleavage  or  segmentation  of  the  ^ggy 
was  observed  long  before  its  meaning  was  understood.  It  seems  to 
have  been  first  definitely  described  in  the  case  of  the  frog's  ^ZZ^  ^Y 
Prevost  and  Dumas  (1824),  though  earlier  observers  had  seen  it;  but 
at  this  time  neither  the  ^gg  nor  its  descendants  were  known  to  be 
cells,  and  its  true  meaning  was  first  clearly  perceived  by  Bergmann, 
Kolliker,  Reichert,  von  Baer,  and  Remak,  some  twenty  years  later. 
The  interpretation  of  cleavage  as  a  process  of  cell-division  was  fol- 
lowed by  the  demonstration  that  cell-division  does  not  begin  with 
cleavage,  but  can  be  traced  back  into  tJie  foregoing  generatioji;  for  the 
egg-cell,  as  well  as  the  sperm-cell,  arises  by  the  division  of  a  cell  pre- 
existing in  the  parent-body.  It  is  therefore  derived  by  direct  descent 
front  an  egg-cell  of  the  foregoing  generation,  and  so  on  ad  infinitum. 
Embryologists  thus  arrived  at  the  conception  so  vividly  set  forth  by 
Virchow  in  1858^  of  an  uninterrupted  series  of  cell-divisions  extend- 
ing backward  from  existing  plants  and  animals  to  that  remote  and 
unknown  period  when  vital  organization  assumed  its  present  form. 
Life  is  a  continuous  stream.  The  death  of  the  individual  involves  no 
breach  of  continuity  in  the  series  of  cell-divisions  by  which  the  life 
of  the  race  flows  onwards.  The  individual  body  dies,  it  is  true,  but 
the  germ-cells  live  on,  carrying  with  them,  as  it  were,  the  traditions 
of  the  race  from  which  they  have  sprung,  and  handing  them  on  to 
their  descendants. 

These  facts  clearly  define  the  problems  of  heredity  and  variation 
as  they  confront  the  investigator  of  the  present  day.  All  theories  of 
evolution  take  as  fundamental  postulates  the  facts  of  variation  and 
heredity  ;  for  it  is  by  variation  that  new  characters  arise  and  by 
heredity  that  they  are  perpetuated.  Darwin  recognized  two  kinds  of 
variation,  both  of  which,  being  inherited  and  maintained  through  the 
conserving  action  of  natural  selection,  might  give  rise  to  a  permanent 
transformation  of  species.  The  first  of  these  includes  congenital  or 
inborn  variations ;  i.e.  such  as  appear  at  birth  or  are  developed 
'*  spontaneously,"  without  discoverable  connection  with  the  activities 
of  the  organism  itself  or  the  direct  effect  of  the  environment  upon  it. 
In  a  second  class  of  variations  are  placed  the  so-called  acquired  char- 

1  See  the  quotation  from  the  original  edition  of  the  Cellidarpathologie  at  the  head  of 
Chapter  II.,  p.  45. 


lO  INTRODUCTION 

acters ;  i.e.  changes  that  arise  in  the  course  of  the  individual  life  as 
the  effect  of  use  and  disuse,  or  of  food,  climate,  and  the  like.  The 
inheritance  of  congenital  characters  is  now  universally  admitted,  but 
it  is  otherwise  with  acquired  characters.  The  inheritance  of  the 
latter,  now  the  most  debated  question  of  biology,  had  been  taken  for 
granted  by  Lamarck  a  half-century  before  Darwin ;  but  he  made  no 
attempt  to  show  how  such  transmission  is  possible.  Darwin,  on  the 
other  hand,  squarely  faced  the  physiological  requirements  of  the  prob- 
lem, recognizing  that  the  transmission  of  acquired  characters  can 
only  be  possible  under  the  assumption  that  the  germ-cell  definitely 
reacts  to  all  other  cells  of  the  body  in  such  wise  as  to  register  the 
changes  taking  place  in  them.  In  his  ingenious  and  carefully  elab- 
orated theory  of  pangenesis,^  Darwin  framed  a  provisional  physio- 
logical hypothesis  of  inheritance  in  accordance  with  this  assumption, 
suggesting  that  the  germ-cells  are  reservoirs  of  minute  germs  or 
gemmules  derived  from  every  part  of  the  body ;  and  on  this  basis  he 
endeavoured  to  explain  the  transmission  both  of  acquired  and  of  con- 
genital variations,  reviewing  the  facts  of  variation  and  inheritance 
with  wonderful  skill,  and  building  up  a  theory  which,  although  it  forms 
the  most  speculative  and  hypothetical  portion  of  his  writings,  must 
always  be  reckoned  one  of  his  most  interesting  contributions  to  science. 
The  theory  of  pangenesis  has  been  generally  abandoned  in  spite 
of  the  ingenious  attempt  to  remodel  it  made  by  Brooks  in  1883.^  In 
the  same  year  the  whole  aspect  of  the  problem  was  changed,  and  a 
new  period  of  discussion  inaugurated  by  Weismann,  who  put  forth 
a  bold  challenge  of  the  entire  Lamarckian  principle."^  "  I  do  not 
propose  to  treat  of  the  whole  problem  of  heredity,  but  only  of  a 
certain  aspect  of  it,  —  the  transmission  of  acquired  characters,  which 
has  been  hitherto  assumed  to  occur.  In  taking  this  course  I  may  say 
that  it  was  impossible  to  avoid  going  back  to  the  foundation  of  all 
phenomena  of  heredity,  and  to  determine  the  substance  with  which 
they  must  be  connected.  In  my  opinion  this  can  only  be  the  sub- 
stance of  the  germ-cells  ;  and  this  substance  transfers  its  hereditary 
tendencies  from  generation  to  generation,  at  first  unchanged,  and 
always  uninfluenced  in  any  corresponding  manner,  by  that  which 
happens  during  the  life  of  the  individual  which  bears  it.  If  these 
views  be  correct,  all  our  ideas  upon  the  transformation  of  species  re- 
quire thorough  modification,  for  the  whole  principle  of  evolution  by 
means  of  exercise  (use  and  disuse)  as  professed  by  Lamarck,  and 
accepted  in  some  cases  bv  Darwin,  entirely  collapses  "    {I.e.,  p.  69). 

1  Variation  of  Animals  and  J*lanis,  Chapter  XXVII. 
^  The  law  of  Heredity,  15altimore,  18S3. 

2  Ueber  VererbiiUi^,  1883.      See  Essays  upon  Heredity,  I.,  l)y  A.  Weismann,  Clarendon 
Press,  Oxford,  1889. 


INTRODUCTION 


I  I 


It  is  impossible,  he  continues,  that  acquired  traits  should  be  trans- 
mitted, for  it  is  inconceivable  that  definite  changes  in  the  body,  or 
**  soma,"  should  so  affect  the  protoplasm  of  the  germ-cells,  as  to  cause 
corresponding  changes  to  appear  in  the  offspring.  How,  he  asks,  can 
the  incre^ed  dexterity  and  power  in  the  hand  of  a  trained  piano- 
player  so  affect  the  molecular  structure  of  the  germ-cells  as  to  produce 
a  corresponding  development  in  the  hand  of  the  child  ?  It  is  a  physi- 
ological impossibility.  If  we  turn  to  the  facts,  we  find,  Vveismann 
affirms,  that  not  one  of  the  asserted  cases  of  transmission  of  acquired 
characters  will  stand  the  test  of  rigid  scientific  scrutiny.  It  is  a 
reversal  of  the  true  point  of  view  to  regard  inheritance  as  taking 
place  from  the  body  of  the  parent  to  that  of  the  child.  The  child 
inherits  from  the  parent  germ-cell,  not  from  the  parent-body,  and  the 
germ-cell  owes  its  characteristics  not  to  the  body  which  bears  it,  but 
to  its  descent  from  a  pre-existing  germ-cell  of  the  same  kind.  Thus 
the  body  is,  as  it  were,  an  offshoot  from  the  germ-cell  (Fig.  4).     As 


S    Line  of  succession. 


(7)  Line  of  inheritance. 

G 

Fig.  4.  —  Diagram  illustrating  Weismann's  theory  of  inheritance. 
G.  The  germ-cell,  which  by  division  gives  rise  to  the  body  or  soma  (6")  and  to  new  germ-cells 
{G)  which  separate  from  the  soma  and  repeat  the  process  in  each  successive  generation. 


far  as  inheritance  is  concerned,  the  body  is  merely  the  carrier  of  the 
germ-cells,  which  are  held  in  trust  for  coming  generations. 

Weismann's  subsequent  theories,  built  on  this  foundation,  have 
given  rise  to  the  most  eagerly  contested  controversies  of  the  post- 
Darwinian  period,  and,  whether  they  are  to  stand  or  fall,  have  played 
a  most  important  part  in  the  progress  of  science.  For  aside  from  the 
truth  or  error  of  his  special  theories,  it  has  been  Weismann's  great 
service  to  place  the  keystone  between  the  work  of  the  evolutionists 
and  that  of  the  cytologists,  and  thus  to  bring  the  cell-theory  and  the 
evolution-theory  into  organic  connection.  It  is  from  this  point  of 
view  that  the  present  volume  has  been  written.  It  has  been  my 
endeavour  to  treat  the  cell  primarily  as  the  organ  of  inheritance  and 
development ;  but,  obviously,  this  aspect  of  the  cell  can  only  be 
apprehended  through  a  study  of  the  general^  phenomena  of  cell-life. 
The  order  of  treatment,  which  is  a  convenient  rather  than  a  strictly 
logical  one,  is  as  follows  :  — 

The  opening  chapter  is  devoted  to  a  general  sketch  of  cell-struct- 


1 2  INTR  OD  UCTION 

ure,  and  the  second  to  the  phenomena  of  cell-division.  The  follow- 
ing three  chapters  deal  with  the  germ-cells,  —  the  third  with  their 
structure  and  mode  of  origin,  the  fourth  with  their  union  in  fertiliza- 
tion, the  fifth  with  the  phenomena  of  maturation  by  which  they  are 
prepared  for  their  union.  The  sixth  chapter  contains  a  critical  dis- 
cussion of  cell-organization,  completing  the  morphological  analysis  of 
the  cell.  In  the  seventh  chapter  the  cell  is  considered  with  reference 
to  its  more  fundamental  chemical  and  physiological  properties  as  a 
prelude  to  the  examination  of  development  which  follows.  The  suc- 
ceeding chapter  approaches  the  objective  point  of  the  book  by  con- 
sidering the  cleavage  of  the  ovum  and  the  general  laws  of  cell-division 
of  which  it  is  an  expression.  The  ninth  chapter,  finally,  deals  with 
the  elementary  operations  of  development  considered  as  cell-functions 
and  with  the  theories  of  inheritance  and  development  based  upon 
them. 

SOME   GENERAL   WORKS    ON   THE   CELL-TH]?ORY 

Bergh,  R.  S.  —  Vorlesungen  liber  die  Zelle  unci  die  einfachen  Gevvebe :    Wiesbaden, 

1894. 
Delage,  Yves.  —  La  Structure  du  Protoplasma  et  les  Theories  sur  Tlieredite  et  les 

grands  Problemes  de  la  Biologie  Generale  :  Paris,  1895. 
Geddes  &  Thompson.  —  Tlie  Evolution  of  Sex  :  New  York,  1890. 
Henneguy,  L.  F.  —  Lemons  sur  la  cellule  :  Paris,  1896. 

Hertwig,  0.  —  Die  Zelle  und  die  Gewebe :  Fischer,  Jena,  1892.     Translation,  pub- 
lished by  Macinillan,  London  and  New  York,  1895. 
Huxley,  T.  H.  —  Review  of  the  Cell-theory  :  British  and  Foreign  Medico-Chirurgical 

Review,  XI L     1853. 
Minot,  C.  S.  —  Human  Embryology:  New  York,  1892. 
Remak,    R.  —  Untersuchungen   iiber   die    Entwicklung   der   Wirbelthiere :    Berlin, 

1850-55. 
Schleiden,  M.  J.  —  Beitrage  zur  Phytogenesis  :  Muller''s  Arc/iiv,  1838.     Translation 

in  Sydenham  Soc,  XH.     London,  1847. 
Schwann,  Th.  —  Mikroscopische    Untersuchungen  iiber  die  Uebereinstimmung  in 

der  Structur  und  dem   Wachsthum   der  Thiere  und   Pflanzen :    Berlin,  1839. 

Translation  in  Sydenham  Soc,  XIL     Londoji,  1847. 
Tyson,  James.  —  The  Cell-doctrine,  2d  ed.     Philadelphia,  1878. 
Virchow,  R.  —  Die  Cellularpathologie  in  ihrer  Begrlindung  auf  physiologische  und 

pathologische  Gewebelehre.     Berlin,  1858. 
Weismann,  A.  —  Essays  on    Heredity.     Translation:    First  series,    Oxford,    1891  ; 

Second  series,  Oxford,  1892. 
Id.  —  The  Germ-plasm.     New  York,  1893. 


CHAPTER  I 

GENERAL  SKETCH  OF  THE  CELL 


"  Wir  haben  gesehen,  class  alle  Organismen  aus  wesentlich  gieichen  Theilen,  namlich  aus 
Zellen  zusammengesetzt  sind,  dass  diese  Zellen  nach  wesentlich  denselben  Gesetzen  sich 
bilden  und  wachsen,  dass  also  diese  Prozesse  iiberall  auch  dutch  dieselben  Krafte  hervorge- 
bracht  vverden  miissen."  Schwann,^ 


The  term  ''  cell  "  is  a  biological  misnomer ;  for  whatever  the  living 
cell  is,  it  is  not,  as  the  word  implies,  a  hollow  chamber  surrounded  by 
solid  walls.  The  term  is  merely  an  historical  survival  of  a  word 
casually  employed  by  the  botanists  of  the  seventeenth  century  to 
designate  the  cells  of  certain  plant-tissues  which,  when  viewed  in 
section,  give  somewhat  the  appearance  of  a  honeycomb.^  The  cells 
of  these  tissues  are,  in  fact,  separated  by  conspicuous  solid  walls 
which  were  mistaken  by  Schleiden,  unfortunately  followed  by 
Schwann  in  this  regard,  for  their  essential  part.  The  living  sub- 
stance contained  within  the  walls,  to  which  Hugo  von  Mohl  gave 
the  n-SLxnQ p7viopIasm^  (1846)  was  at  first  overlooked  or  was  regarded 
as  a  waste-product,  a  view  based  upon  the  fact  that  in  many  im- 
portant plant-tissues  such  as  cork  or  wood  it  may  wholly  disappear, 
leaving  only  the  lifeless  walls.  The  researches  of  Bergmann, 
Kolliker,  Bischoff,  Cohn,  Max  Schultze,  and  many  others,  showed, 
however,  that  some  kinds  of  cells,  for  example,  the  corpuscles  of 
the  blood,  are  naked  masses  of  living  protoplasm  not  surrounded  by 
walls,  —  a  fact  which  proves  that  not  the  wall,  but  the  cell-contents, 
is  the  essential  part,  and  must  therefore  be  the  seat  of  life.  It  was 
found  further  that  with  the  possible  exception  of  some  of  the  lowest 
forms  of  life,  such  as  the  bacteria,  the  protoplasm  invariably  contains 
a  definite  rounded  body,  the  nucleus,^  which  in  turn  may  contain  a  still 

1  Untersiichmigen,  p.  227,  1 839. 

2  The  word  seems  to  have  been  first  employed  by  Robert  Hooke,  in  1665,  to  designate 
the  minute  cavities  observed  in  cork,  a  tissue  which  he  described  as  made  up  cf  "  little 
boxes  or  cells  distinct  from  one  another  "  and  separated  by  solid  walls. 

•^  The  same  word  had  been  used  by  Purkyne  some  years  before  (1840)  to  designate  the 
formative  material  of  young  animal  embryos. 
'*  First  described  by  Robert  Brown  in  1833. 

13 


H 


GENERAL   SKETCH   OF   THE    CELL 


smaller  body,  the  nucleolus.  Thus  the  cell  came  to  be  defined  by 
Max  Schultze  and  Leydig  as  a  mass  of  protoplasm  containing 
a  juiclens,  a  morphological  definition  which  remains  sufficiently  satis- 
factory even  at  the  present  day.  Nothing  could  be  less  appropriate 
than  to  call  such  a  body  a  "  cell  "  ;  yet  the  word  has  become  so  firmly 
established  that  every  effort  to  replace  it  by  a  better  has  failed,  and 
it  probably  must  be  accepted  as  part  of  the  established  nomenclature 
of  science.^ 

Attraction-sphere  enclosing  two  centrosomes. 


Plasmosome  or 
true  nucleolus. 

Chromatin- 

network, 


Nucleus  \ 


I  Linin-network. 
Karyosome  or 


net-knot.  — ' 


Plastids     lying 
cytoplasm. 


the 


Vacuole. 


Lifeless  bodies  (meta- 
plasm)  suspended  in 
the  cytoplasmic  reticu- 
lum. 


Fig.  5.  —  Diagram  of  a  cell.     Its  basis  consists  of  a  thread-work  (mitome,  or  reticuluvi)  com- 
posed of  minute  granules  {microsomes)  and  traversing  a  transparent  ground-substance. 


A.  General  Morphology  of  the  Cell 


The  cell  is  a  rounded  mass  of  protoplasm  which  in  its  simplest 
form  is  approximately  spherical.  This  form  is,  however,  seldom 
realized  save  in  isolated  cells  such  as  the  unicellular  plants  and 
animals  or  the  egg-cells  of  the  higher  forms.  In  vastly  the  greater 
number  of  cases  the  typical  spherical  form  is  modified  by  unequal 
growth  and  differentiation,  by  active  movements  of  the  cell-substance, 
or   by   the    mechanical    pressure    of    surrounding    structures.      The 

J  Sachs  has  proposed  the  convenient  word  energid  {Fiora,  '92,  p.  57)  to  designate  the 
essential  living  part  of  the  cell,  i.e.  the  nucleus  with  that  portion  of  the  active  cytoplasm 
that  falls  within  its  sphere  of  influence,  the  two  forming  an  organic  unit  both  in  a  morpho- 
logical and  in  a  physiological  sense.  It  is  to  be  regretted  that  this  convenient  and  appro- 
priate term  has  not  come  into  general  use.     (See  also  Llora,  '95,  p.  405.) 


GENERAL   MORPHOLOGY   OF   THE    CELL 


protoplasm  which  forms  its  living  basis  is  a  viscid,  translucent, 
granular  substance,  often  forming  a  network  or  sponge-like  structure 
extending  through  the  cell-body  and  showing  various  structural 
modifications  in  different  regions  and  under  different  physiological 
states  of  the  cell.  Besides  the  living  protoplasm  the  cell  almost 
invariably  contains  various  lifeless  bodies  suspended  in  the  meshes 
of  the  network ;  examples  of  these  are  food-granules,  pigment-bodies, 
drops  of  oil  or  water,  and  excretory  matters.  These  bodies  play  a 
purely  passive  part  in  the  activities  of  the  cell,  being  either  reserve 
food-matters  destined  to  be  absorbed  and  built  up  into  the  living 
substance,  or  by-products  formed  from  the  protoplasm  as  waste 
matters,  or  in  order  to  play  some  role  subsidiary  to  the  actions  of 
the  protoplasm  itself.  The  lifeless  inclusions  in  the  protoplasm  have 
been  collectively  designated  as  metaplasni  (Hanstein)  in  contradis- 
tinction to  the  living  protoplasm  ;  but 
this  convenient  term  is  not  in  general 
use.  Among  the  lifeless  products  of 
the  protoplasm  must  be  reckoned 
also  the  cell-zuall  or  membrane  by 
which  the  cell-body  may  be  sur- 
rounded ;  but  it  must  be  remembered 
that  the  cell-wall  in  many  cases  arises 
by  a  direct  transformation  of  the 
protoplasmic  substance,  and  that  it 
often  retains  the  power  of  growth  by 
intussusception  like  living  matter. 

In  all  save  a  few  of  the  lowest  and 
simplest  forms,  perhaps  even  in  them, 
the  protoplasmic  substance  is  differ- 
entiated into  two  very  distinct  parts, 
viz.,  the  cell-body,  forming  the  princi- 
pal mass  of  the  cell,  and  a  smaller 
body,  the  nucleus,  which  lies  in  its 
interior  (Fig.  5).  Both  structurally 
and  chemically  these  two  parts  show 
differences  of  so  marked  and  constant 
a  character  that  they  must  be  re- 
garded as  the  most  important  of  all 
protoplasmic  differentiations.  The 
nuclear  substance  is  therefore  often  designated  as  nucleoplasm  or 
karyoplasm  ;  that  of  the  cell-body  as  cytoplasm  (Strasburger).  Some 
of  the  foremost  authorities,  however,  among  them  Oscar  Hertwig,  re- 
ject this  terminology  and  use  the  word  "  protoplasm  "  in  its  historic 
sense,  applying  it  solely  to  the  cytoplasm  or  substance  of  the  cell-body. 


a         C 

Fig.  6.  —  A  resting  cell  {spertnatogo- 
tiium)  from  the  testis  of  the  salamander, 
showing  the  typical  parts.  Above,  the  large 
nucleus,  with  scattered  masses  of  chro- 
matin, linin-network  and  membrane. 
Around  it,  the  cytoplasmic  thread-work. 
Below,  the  attraction-sphere  {a)  and  cen- 
trosome  {c).     [After  Rawitz.] 


GENERAL   SKETCH  OF   THE    CELL 


At  a  first  examination  the  nucleus  appears  to  be  a  perfectly  dis- 
tinct body  suspended  in  the  cytoplasm.  Most  of  the  latest  researches 
point,  however,  to  the  conclusion  that  nucleus  and  cytoplasm  are 
pervaded  by  a  common  structural  basis,  morphologically  continuous 


M: 


J^-J.. 


.<>\^ 


C: 


B 


C  D 

Fig-  7-  —  Various  cells  showing  the  typical  parts. 

A.  From  peritoneal  epithelium  of  the  salamander-larva.  Two  centrosomes  at  the  right. 
Nucleus  showing  net-knots.     [FLEMMING.] 

B.  Spermatogonium  of  frog.  Attraction-sphere  (aster)  containing  a  single  centrosome. 
Nucleus  with  a  single  plasmosome.     [Hermann.] 

C.  Spinal  ganglion-cell  of  frog.  Attraction-sphere  near  the  centre,  containing  a  single  centro- 
some with  several  centrioles.     [Lenhoss^K.] 

Z).  Spermatocyte  of  /'/•^/^^j.  Nucleus  in  the  spireme-stage.  Centrosome  single  ;  attraction- 
sphere  containing  rod-shaped  bodies.     [Hermann.] 

under  certain  conditions  from  one  to  the  other,  and  that  both  are  to 
be  regarded  as  specially  differentiated  areas  in  that  basis.^    The  terms 

1  The  fact  that  the  nucleus  may  move  actively  through  the  cytoplasm,  as  occurs  iiurin«; 
the  fertilization  of  the  egg  and  in  some  other  cases,  seems  to  show  that  the  morphological 
continuity  may  at  times  be  interrupted. 


STRUCTURAL   BASIS   OF  PROTOPLASM  IJ 

''nucleus"  and  "cell-body"  are  therefore  only  topographical  expres- 
sions, and  in  a  measure  the  same  is  true  of  the  terms  **karyoplasm  "  and 
"  cytoplasm."  The  latter,  however,  acquire  a  special  significance  from 
the  fact  that  there  is  on  the  whole  a  definite  chemical  contrast  be- 
tween the ''nuclear  substance  and  that  of  the  cell-body,  the  former 
being  characterized  by  the  abundance  of  a  substance  rich  in  phos- 
phorus known  as  nuclein,  while  the  latter  contains  no  true  nuclein  and 
•is  especially  rich  in  proteids  and  related  substances  (nucleo-albumins, 
albumins,  globulins,  and  others),  which  contain  a  much  lower  per- 
centage of  phosphorus. 

The  differentiation  of  the  protoplasmic  substance  into  nucleus  and 
cytoplasm  is  a  fundamental  character  of  the  cell,  both  in  a  morpho- 
logical and  in  a  physiological  sense  ;  and,  as  will  appear  hereafter, 
there  is  reason  to  believe  that  it  is  in  a  measure  the  expression  of 
a  corresponding  localization  of  the  operations  of  constructive  and 
destructive  metabolism  which  lie  at  the  basis  of  the  individual  cell- 
life.  A  third  element,  the  centrosome  (Figs.  5-7),  present  in  many 
if  not  in  all  cells,  is  especially  concerned  with  the  process  of  division 
and  cell-reproduction.  Recent  research  has  rendered  it  probable  that 
in  point  of  morphological  persistency  the  centrosome  is  comparable 
with  the  nucleus  ;  but  this  conclusion  is  not  yet  definitely  established. 


B.     Structural  Basis  of  Protoplasm 

As  ordinarily  seen  under  moderate  powers  of  the  microscope  proto- 
plasm shows  no  definite  structural  organization.  A  more  precise  ex- 
amination under  high  powers,  especially  after  treatment  with  suitable 
fixing  and  staining  reagents,  reveals  the  fact  that  both  nucleus  and 
cytoplasm  possess  a  complicated  structure.  Regarding  the  pre- 
cise nature  of  this  structure  opinion  still  differs.  According  to  the 
view  most  widely  held,'  one  of  its  essential  features  is  the  presence 
of  two  constituents,  one  of  which,  the  ground-substance,  cyto- 
lympJi,  or  enchylema,  is  more  liquid,  while  the  other,  the  spongio- 
plasm  or  reticulum,  is  of  firmer  consistency,  and  forms  a  sponge-like 
network  or  alveolar  structure  extending  everywhere  through  the  more 
liquid  portion.  At  the  present  time  it  seems  probable  that  the 
more  solid  portion  is  the  more  active  and  is  perhaps  to  be  identified 
as  the  living  substance  proper,  the  ground-substance  being  passive  ; 
but  the  reverse  of  this  view  is  maintained  by  Leydig,  Schafer,  and 
some  others.  The  most  elaborate  and  painstaking  investigation  has 
moreover  failed  to  determine  with  absolute  certainty  even  the  physi- 
cal configuration  of  the  network. 

Biitschli    and    a    considerable    school    of    followers    among    both 


i8 


GENERAL  SKETCH   OF   THE    CELL 


zoologists  and  botanists  regard  protoplasm  as  essentially  a  liquid,  or 
rather  a  mixture  of  liquids,  which  forms  a  foam-like  alveolar  structure^ 
like  an  emulsion,  in  which  the  firmer  portion  forms  the  walls  of  sepa- 
rate chambers,  filled  with  the  more  liquid  substance  (Fig.   8).     By 


A 


Fig.  8.  —Alveolar  or  foam-structure  of  protoplasm,  according  to  Butschli.     [BiJTSCHLl.] 
A.  Epidermal  cell  of  the  earthworm.     D.  Aster,  attraction-sphere,  and  centrosome  from  sea- 
urchin    egg.      C.    Intra-capsular    protoplasm   of   a    radiolarian    {Thalassicolld)    with   vacuoles. 
D.  Peripheral  cytoplasm  of  sea-urchin  egg.     E.  Artificial  emulsion  of  olive-oil,  sodium  chloride, 
and  water. 


special  local  modifications  of  this  structure  all  the  parts  of  the  cell  are 
formed.  Butschli  has  shown  that  artificial  emulsions,  variously  pre- 
pared, may  show  under  the  microscope  a  marvellously  close  resem- 

1  "  Wabenstruklur. " 


STRUCTURAL    BASIS    OF  PROTOPLASM  I9 

blance  to  actual  protoplasm,  and  that  drops  of  oil-emulsions  suspended 
in  water  may  even  exhibit  amoeboid  changes  of  form. 

Opposed  to  BiJtschli's  conception  is  the  view,  first  clearly  set  forth 
by  FromQiann  and  Arnold  ('65-67),  and  now  maintained  by  such 
authorities  as  Flemming,  Van  Beneden,  Strasburger,  and  perhaps  the 
greater  number  of  contemporary  investigators,  that  the  more  solid 
portion  consists  of  coherent  tJircads  which  extend  through  the  ground- 
substance,  either  separately  or  connected  by  branches  to  form  a  mesh- 
work  like  the  fibres  of  a  sponge  (Figs.  7,  9). 

In  the  present  state  of  the  subject  it  is  difficult,  indeed,  impossible, 
to  decide  which  of  these  opposing  views  should  be  accepted  ;  for  the 
evidence  is  very  strong  that  each  expresses  a  part  of  the  truth.  It  is 
generally  admitted  that  such  an  alveolar  structure  as  Biitschli  de- 
scribes is  characteristic  of  many  unicellular  forms,  and  occurs  in 
many  higher  forms  where  the  cell-substance  is  filled  with  vacuoles  or 
with  solid  inclusions  such  as  starch-grains  or  deutoplasm-spheres. 
In  the  latter  case  the  structure  has  been  termed  "pseudo-alveolar" 
(Reinke);  but  it  remains  to  be  seen  whether  there  is  any  real  dis- 
tinction between  this  and  the  true  alveolar  structure  described  by 
Biitschli.  On  the  other  hand  the  evidence  of  true  fibrillar  or  reticular 
structure  in  many  tissue-cells,  especially  during  cell-division,  is  very 
convincing  ;  and  my  own  observations  have  led  me  to  regard  this 
structure  as  the  more  typical  and  characteristic.  For  descriptive  pur- 
poses I  shall  accordingly  adopt  the  terms  of  the  fibrillar  or  reticular 
hypothesis,  designating  the  more  solid  portion  of  protoplasm  as  the 
thread-work  or  reticidum  ("Geriistwerk,"  "Fadenwerk"  of  German 
writers)  in  contradistinction  to  the  more  liquid  groiuid-substance.  It 
should  be  clearly  understood,  however,  that  these  terms  are  used  only 
as  a  matter  of  convenience,  and  are  not  meant  to  exclude  the  possibility 
that  the  "fibres"  or  the  "reticulum"  may  in  many  cases  be  open  to 
Biitschli's  interpretation. 

From  a  theoretical  point  of  view  the  finer  structure  of  the  network 
is  a  question  of  very  great  interest  ^d  importance.  The  earlier 
investigators,  such  as  Virchow  and  Max  Schultze,  failed  to  observe 
the  thread-work,  and  described  protoplasm  as  consisting  of  a  clear 
homogeneous  basis  in  which  were  embedded  numerous  granules. 
Even  at  the  present  time  a  similar  view  is  held  by  a  few  investi- 
gators, more  especially  among  botanists  {e.g.,  Berthold,  Schwarz), 
who  regard  the  thread-work  either  as  an  artificial  effect  produced 
by  reagents,  or,  if  normal,  as  an  inconstant  and  hence  unimportant 
feature.  The  best  and  most  careful  recent  studies  on  proto- 
plasm have,  however,  yielded  very  convincing  evidence  that,  what- 
ever be  the  precise  configuration  of  the  protoplasmic  reticulum, 
it  is  not  only  a  normal  structure,  but  one  of  very  wide  occurrence. 


20 


GENERAL   SKETCH  OF   THE    CELL 


U-T*^.. 


v: 


'>■-. .. 


( 


A 


-^< 


mm- 


L-::^ 


/) 


i: 


Fig.  9.  —  Living  cells  of  salamander-larva.     [Flkmming.] 
A.  Group  of  epidermal  cells  at  different  foci,  showing  protoplasmic  bridges,  nuclei,  and  cyto- 
plasmic fibrillae;  the  central  cell  with  nucleus  in  the  spireme-stage.      B.  Connective  tissue-cell. 

C.  Epidermal  cell  in  early  mitosis  (segmented  spireme)  surrounded  by  protoplasmic  bridges. 

D.  Dividing-cell.    E.F.  Cartilage-cells  with  cytoplasmic  fiibrillai  (the  latter  somewhat  exaggerated 
in  the  plate). 


STRUCTURAL   BASIS    OF  PROTOPLASM  21 

These  studies  have  raised  interesting  problems  regarding  the  signifi- 
cance of  the  granules  described  by  the  early  observers.  Many  of  the 
granules,  especially  the  larger  and  more  obvious  of  them,  are  unques- 
tionably irfert  bodies,  such  as  reserve  food-matters,  suspended  in  the 
meshvvork.  Others  are  nodes  of  the  network  or  optical  sections  of 
the  threads.  But  there  is  some  reason  to  believe  that,  apart  from 
these  appearances,  discrete  living  particles  may  form  a  constant 
and  essential  structural  feature  of  the  protoplasmic  thread.  These 
particles,  now  generally  known  as  microsomes}  are  embedded  in  the 
threads  of  the  network,  and  are  sometimes  so  closely  and  regularly  set 
as  irresistibly  to  suggest  the  view  that  they  are  definite  structural  ele- 
ments out  of  which  the  thread  is  built.  More  than  this,  their  behaviour 
is  in  some  cases  such  as  to  have  led  to  the  hypothesis  long  since 
suggested  by  Henle  ('41)  and  at  a  later  period  developed  by  Bechamp 
and  Estor,  by  Maggi  and  especially  by  Altmann,  that  microsomes  are 
actually  organic  units  or  bioblasts,  capable  of  assimilation,  growth,  and 
division,  and  hence  to  be  regarded  as  elementary  units  of  structure 
standing:  between  the  cell  and  the  ultimate  molecules  of  living:  matter. 
And  thus  the  theory  of  genetic  continuity  expressed  by  Redi  in  the 
aphorism  ^^  omne  vivtun  ex  vivo^''  reduced  by  Virchow  to  '^  omnis 
cellida  e  celhila,''  finally  appears  in  the  writings  of  Altmann  as  ''  omne 
gramdnm  e  grajuilo!  "  ^ 

Altmann's  premature  generalization  rests  upon  a  very  insecure 
foundation  and  has  been  received  with  just  scepticism.  That  the  cell 
consists  of  more  elementary  units  of  organization  is  nevertheless  in- 
dicated by  a  priori  evidence  so  cogent  as  to  have  driven  many  of  the 
foremost  leaders  of  biological  thought  into  the  belief  that  such  units 
must  exist,  whether  or  not  the  microscope  reveals  them  to  view. 
Among  those  who  have  accepted  this  conception  in  one  form  or 
another  are  numbered  such  men  as  Spencer,  Darwin,  Beale,  Haeckel, 
Michael  Foster,  Nageli,  De  Vries,  Wiesner,  Roux,  Weismann,  Oscar 
Hertwig,  Verworn,  and  Whitman.  The  modern  conception  of  ultra- 
cellular  units,  ranking  between  the  molecule  and  the  cell,  was  first 
definitely  suggested  by  Briicke  ('61),^  only,  however,  to  be  rejected 
as  without  the  support  of  facts,  though  this  eminent  physiologist 
insisted  that  the  cell  must  possess  a  more  complicated  organization 
than  that  revealed  by  the  best  microscopes  of  his  time.  It  was  soon 
afterwards  taken  up  by  Herbert  Spencer,  and  elaborated  into  the 
theory  of  physiological  units  by  which  he  endeavoured  to  explain 
the  phenomena  of  regeneration,  development,  and  heredity.     Darwin 

1  Hanstein  ('82). 

2  Die  Elementarorganismen,  Leipsic,  1894,  p.  155. 

3  For  a  review  of  speculations  in  the  same  direction  by  Buffon  and  other  early  writers  see 
Yves  Delagc  ('95). 


22  GENERAL    SKETCH  OF   THE    CELL 

too,  in  his  celebrated  hypothesis  of  pangenesis,  adopted  a  nearly 
related  conception,  which  as  remodelled  by  De  Vries  twenty  years 
afterwards  ('89)  forms  the  basis  of  the  theories  of  development 
maintained  by  such  leaders  of  biological  research  as  Weismann  and 
Hertwig.  The  same  view  appears  in  a  different  form  in  the  writ- 
ings of  Nageli,  Wiesner,  Foster,  Verworn,  and  many  other  morpholo- 
gists  and  physiologists.^ 

An  hypothesis  backed  by  such  authority  and  based  on  evidence 
drav^rn  from  sources  so  diverse  cannot  be  lightly  rejected.  We  are 
compelled  by  the  most  stringent  evidence  to  admit  that  the  ultimate 
basis  of  living  matter  is  not  a  single  chemical  substance,  but  a  mixture 
of  many  substances  tJiat  are  self-pcrpetiiating  without  loss  of  their 
specific  character.  The  open  question  is  whether  these  substances 
are  localized  in  discrete  morphological  bodies  aggregated  to  form  the 
cell  somewhat  as  cells  are  aggregated  to  form  tissues  and  organs, 
and  whether  such  bodies,  if  they  exist,  lie  within  the  reach  of  the 
microscope.  Altmann's  identification  of  the  "granulum"  as  such  a 
body  is  undoubtedly  premature ;  it  is  certain  that  his  description  of 
cell-structures  from  this  point  of  view  is  often  very  inaccurate  ;  it 
is  extremely  doubtful  how  far  the  granules  or  microsomes  are  normal 
structures,  and  how  far  they  are  artefacts  produced  by  the  coagulat- 
ing effect  of  the  reagents.  It  is  nevertheless  certain,  as  will  be 
shown  in  Chapter  VI.,  that  at  least  one  part  of  the  cell,  namely  the 
nucleus,  actually  consists  of  self-propagating  units  of  a  lower  order 
than  itself,  and  there  is  some  ground  for  regarding  the  cyto-micro- 
somes  in  the  same  light. 


C.     The  Nucleus 

A  fragment  of  a  cell  deprived  of  its  nucleus  may  live  for  a  con- 
siderable time  and  manifest  the  power  of  co-ordinated  movement 
without  perceptible  impairment.  Such  a  mass  of  protoplasm  is,  how- 
ever, devoid  of  the  powers  of  assimilation,  growth,  and  repair,  and 
sooner  or  later  dies.  In  other  words,  those  functions  that  involve 
destructive  metabolism  may  continue  for  a  time  in  the  absence  of 
the  nucleus  ;  those  that  involve  constructive  metabolism  cease  with 
its  removal.  There  is,  therefore,  strong  reason  to  believe  that  the 
nucleus  plays  an  essential  part  in  the  constructive  metabolism  of  the 

^  The  following  list  includes  only  some  of  the  various  names  that  have  been  given  to 
these  hypothetical  units  by  niorlern  writers  :  Physiological  unifs  (Spencer) ;  gemmtiles 
(Darwin);  pangens  (De  Vries);  plasomes  (Wiesner);  micelUc  (Nageli);  plastidules 
(Haeckel  and  Elssberg) ;  inotagmata  (P]ngelmann) ;  biophores  (Weismann);  hioblasts 
(Beale);  sotnacules  (Foster);  idioblasts  (Hertwig);  idiosomes  (Whitman);  biogens  (Ver- 
worn);   tnicrozymos  (Rechamp  and  Estor)  ;  gemnue  (TIaacke). 


THE   NUCLEUS  23 

cell,  and  through  this  is  especially  concerned  with  the  formative  proc- 
esses involved  in  growth  and  development.  For  these  and  many 
other  reasons,  to  be  discussed  hereafter,  the  nucleus  is  generally  re- 
garded as  a  controlling  centre  of  cell-activity,  and  hence  a  primary 
factor  in  growth,  development,  and  the  transmission  of  specific  quali- 
ties from  cell  to  cell,  and  so  from  one  generation  to  another. 


I.    General  Structure 

The  cell-nucleus  passes  through  two  widely  different  phases,  one 
of  which  is  characteristic  of  cells  in  their  ordinary  or  vegetative  con- 
dition, while  the  other  only  occurs  during  the  complicated  changes 
involved  in  cell-division.  In  the  first  phase,  falsely  characterized 
as  the  "  resting  state,"  the  nucleus  usually  appears  as  a  rounded 
sac-like  body  surrounded  by  a  distinct  membrane  and  containing  a 
conspicuous  irregular  network  (Figs.  5,  7,  10).  Its  form,  though 
subject  to  variation,  is  on  the  whole  singularly  constant,  and  shows 
no  definite  relation  to  that  of  the  cell  in  which  it  lies.  Typically 
spherical,  it  may,  in  certain  cases,  assume  an  irregular  or  amoeboid 
form,  may  break  up  into  a  group  of  more  or  less  completely  sepa- 
rated lobes  (polymorphic  nuclei),  or  may  be  perforated  to  form  an 
irregular  ring  (Fig.  11,  D).  It  is  usually  very  large  in  gland-cells 
and  others  that  show  a  very  active  metabolism,  and  in  such  cases 
its  surface  is  sometimes  increased  by  the  formation  of  complex 
branches  ramifying  through  the  cell  (Fig.  11,  E).  Interesting  modi- 
fications of  the  nucleus  occur  in  the  unicellular  forms.  In  the 
ciliate  Infusoria  the  body  contains  nuclei  of  two  kinds,  viz.  a  large 
niacromicleiis  and  one  or  more  smaller  inicroniiclei.  The  first  of 
these  shows  a  remarkable  diversity  of  structure  in  different  forms, 
being  often  greatly  elongated  and  sometimes  showing  a  moniliform 
structure  like  a  string  of  beads.  In  Trachelocerca  and  some  other 
Infusoria,  according  to  Gruber  ('84),  the  nucleus  is  not  a  single  definite 
body,  but  is  represented  by  minute  granules  scattered  throughout  the 
cell-substance  (Fig.  12) ;  Butschli  describes  somewhat  similar  diffused 
nuclei  in  some  of  the  Flagellates,  and  in  the  Bacteria. 

In  the  ordinary  forms  of  nuclei  in  their  resting  state  the  following 
structural  elements  may  as  a  rule  be  distinguished  (Figs.  5,6,7, 10, 11): — 

a.  The  nuclear  membrane,  a  well-defined  delicate  wall  which  gives 
the  nucleus  a  sharp  contour  and  differentiates  it  clearly  from  the 
surrounding  cytoplasm. 

b.  The  nuclear  reticuhwi.  This,  the  most  essential  part  of  the 
nucleus,  forms  an  irregular  branching  network  or  reticulum  which 
consists  of  two  very  different   constituents.     The  first  of  these,  the 


24 


GENERAL   SKETCH  OF   THE    CELL 


nuclear  substance  par  excellence,  is  known  as  cJiromatin  (Flemming) 
on  account  of  its  very  marked  staining  capacity  when  treated  with 
various  dyes.  In  some  cases  the  chromatin  forms  a  nearly  continu- 
ous network,  but  it  often  appears  in  the  form  of  more  or  less  detached 
rounded  granules  or  irregular  bodies.  The  second  constituent  is  a 
transparent  substance,  invisible  until  after   treatment   by  reagents, 

known  as  /////;/  (Schwarz).  This 
substance,  which  is  probably  of 
the  same  nature  as  the  cyto- 
plasmic network  outside  the 
nucleus,  surrounds  and  supports 
the  chromatin,  and  thus  forms 
the  basis  of  the  nuclear  net- 
work. 

c.  The  nucleoli,  one  or  more 
larger  rounded  or  irregular 
bodies,  suspended  in  the  net- 
work, and  staining  intensely 
with  many  dyes  ;  they  may  be 
absent.  The  bodies  known  by 
this  name  are  of  at  least  two 
different  kinds.  The  first  of 
these,  the  so-called  true  nucleoli 
or  plasmosornes  (Figs.  5,  7,  B, 
10),  are  of  spherical  form,  and 
by  treatment  with  differential 
stains  such  as  haematoxylin  and 
eosin  are  found  to  consist  typi- 
cally of  a  central  mass  staining 
like  the  cytoplasm,  surrounded 
by  a  shell  which  stains  like 
chromatin.  Those  of  the  other 
form,  the  **  net-knots "  (Netz- 
knoten),  or  karyosovies,  are  either 
spherical  or  irregular  in  form, 
stain  like  the  chromatin,  and 
appear  to  be  no  more  than  thickened  portions  of  the  chromatic 
network  (Figs.  5,  7,  A,  10).  Besides  the  nucleoli  the  nucleus  may 
in  exceptional  cases  contain  the  centrosome  (p.  225),  which  has 
undoubtedly  been  confounded  in  some  instances  with  a  true  nucleolus 
or  plasmosome.^     There  is  strong  evidence  that  the  true  nucleoli  are 

1  Flemming  first  called  attention  to  the  chemical  difference  between  the  true  nucleoli  and 
the  chromatic  reticulum  ('82,  pp.  138,  163)  in  animal  cells,  and  Zacharias  soon  afterwards 
studied   more  closely  the   difference  of  staining-reaction  in    plant-cells,   showing  tliat    the 


Fig.  10.  —  Two  nuclei  from  the  crypts  of 
Lieberkiihn  in  the  salamander.    [Heidenhain.] 

The  character  of  the  chromatin-network 
{basichromatin)  is  accurately  shown.  The  upper 
nucleus  contains  three  plasmosornes  or  true 
nucleoli ;  the  lower,  one.  A  few  fine  linin-threads 
{oxy chromatin)  are  seen  in  the  upper  nucleus 
running  off  from  the  chromatin-masses.  The 
clear  spaces  are  occupied  by  the  ground-sub- 
stance. 


THE  NUCLEUS 


25 


relatively  passive  bodies  that  represent  accumulations  of  reserve- 
substance  or  by-products,  and  play  no  direct  part  in  the  nuclear 
activity  (p.  93). 

d.    The  ground-substance ^  rmclear  sap,  or  karyolymph,  a  clear  sub- 


C  E 

Fig.  II.  —  Special  forms  of  nuclei. 

A.  Permanent  spireme-nucleus,  salivary  gland  of  C/iironot?nts  larva.  Chromatin  in  a  single 
thread,  composed  of  chromatin-discs  (chromomeres),  terminating  at  each  end  in  a  true  nucleolus 
or  plasmosome.     [Balkiani.] 

B.  Permanent  spireme-nuclei,  intestinal  epithelium  of  dipterous  larva  Ptychoptera.  [Van 
Gehuchten.]     C.  The  same,  side  view. 

D.  Polymorphic  ring-nucleus,  giant-cell  of  bone-marrow  of  the  rabbit ;  c,  a  group  of  centro- 
somes  or  centrioles.     [Heidknhain.] 

E.  Branching  nucleus,  spinning-gland  of  butterfly  larva  {Pieris).     [Korschelt.] 

stance  occupying  the  interspaces  of  the  network  and  left  unstained 
by  many  dyes  which  colour  the  chromatin  intensely.      Until  recently 

former  are  especially  coloured  by  alkaline  carmine  solutions,  the  latter  by  acid  solutions. 
Still  later  studies  by  Zacharias,  and  especially  by  Heidenhain,  show  that  the  medullary 
substance  (pyrenin)  of  true  nuclei  is  coloured  by  acid  anilines  and  other  plasma  stains, 
while  the  chromatin  has  a  special  affinity  for  basic  anilines.     Cf.  p.  242. 


26 


GENERAL   SKETCH   OF   THE    CELL 


the  ground-substance  has  been  regarded  as  a  fluid  or  semi-fluid,  but 
recent  researches  by  Reinke  and  others  have  thrown  doubt  on  this 
view,  as  described  at  p.  28. 

The  configuration  of  the  chromatic  network  varies  greatly  in  dif- 
ferent cases.  It  is  sometimes  of  a  very  loose  and  open  character, 
as  in  many  epithelial   cells    (Fig.    i);    sometimes   extremely  coarse 

and  irregular,  as  in  leucocytes  (Fig. 
10)  ;  sometimes  so  compact  as  to 
appear  nearly  or  quite  homogeneous, 
as  in  the  nuclei  of  spermatozoa  and 
in  many  Protozoa.  In  some  cases 
the  chromatin  does  not  form  a  net- 
work, but  appears  in  the  form  of  a 
thread  closely  similar  to  the  spireme- 
stage  of  dividing  nuclei  (cf.  p.  47). 
The  most  striking  case  of  this  kind 
occurs  in  the  salivary  glands  of  dip- 
terous larvae  {Chirononius)^  where,  as 
described  by  Balbiani,  the  chromatin 
has  the  form  of  a  single  convoluted 
thread,  composed  of  transverse  discs 
and  terminating  at  each  end  in  a 
large  nucleolus  (Fig.  11,  A),  Some- 
what similar  nuclei  (Fig.  11,  B)  occur 
in  various  glandular  cells  of  other 
insects  (Van  Gehuchten,  Gilson),  and 
also  in  the  young  ovarian  eggs  of  cer- 
tain animals  (cf.  p.  193).  In  certain 
gland-cells  of  the  marine  isopod  Ani- 
locra  it  is  arranged  in  regular  rosettes 
(Vom  Rath).  Rabl,  followed  by  Van 
Gehuchten,  Heidenhain,  and  others, 
has  endeavoured  to  show  that  the 
nuclear  network  shows  a  distinct 
polarity,  the  nucleus  having  a  "  pole  " 
towards  which  the  principal  chromatin-threads  converge,  and  near 
which  the  centrosome  lies.^  In  many  nuclei,  however,  no  trace  of 
such  polarity  can  be  discerned. 

The  network  may  undergo  great  changes  both  in  physical  con- 
figuration and  in  staining  capacity  at  different  periods  in  the  life 
of  the  same  cell,  and  the  actual  amount  of  chromatin  fluctuates, 
sometimes  to  an  enormous  extent.      Embryonic  cells  are  in  general 


Fig.  12. — An  infusorian,  Trachelo- 
cerca,  with  diffused  nucleus  consisting  of 
scattered  chromatin-granules.  [Gruber.] 


1  ( "f.  the  polarity  of  the  cell,  p.  38. 


THE  NUCLEUS 


27 


characterized  by  the  large  size  of  the  nucleus;  and  Zacharias  has 
shown  in  the  case  of  plants  that  the  nuclei  of  meristem  and  other 
embryonic  tissues  are  not  only  relatively  large,  but  contain  a  larger 
percentage^of  chromatin  than  in  later  stages.  The  relation  of  these 
changes  to  the  physiological  activity  of  the  nucleus  is  still  imperfectly 
understood.^ 

A  description  of  the  nucleus  during  division  is  deferred  to  the  fol- 
lowing chapter. 


2.    Finer  Structure  of  the  Nucleus 

Many  recent  researches  indicate  that  some  at  least  of  the  nuclear 
structures  are  aggregates  of  more  elementary  morphological  bodies, 
though  there  is  still  no  general  agreement  regarding  their  nature  and 
relationships.  The  most  definite  evidence  in  this  direction  relates 
to  the  chromatic  network.  In  the  stages  preparatory  to  division 
this  network  revolves  itself  into  a  definite  number  of  rod-shaped 
bodies  known  as  chromosomes  (Fig.  16),  which  split  lengthwise  as 
the  cell  divides.  These  bodies  arise  as  aggregations  of  minute 
rounded  bodies  or  microsomes  to  which  various  names  have  been 
given  {cJiromomeres,  Fol ;  ids,  Weismann).  They  are  as  a  rule 
most  clearly  visible  and  most  regularly  arranged  during  cell-division, 
when  the  chromatin  is  arranged  in  a  thread  {spireme),  or  in  separate 
chromosomes  (Figs.  7,  D,  38,  B)  ;  but  in  many  cases  they  are  dis- 
tinctly visible  in  the  reticulum  of  the  "resting"  nucleus  (Fig.  39). 
It  is,  however,  an  open  question  whether  the  chromatin-granules 
of  the  reticulum  are  individually  identical  with  those  forming  the 
chromosomes  or  the  spireme-thread.  The  larger  masses  of  the 
reticulum  undoubtedly  represent  aggregations  of  such  granules,  but 
whether  the  latter  completely  fuse  or  remain  always  distinct  is 
unknown.  Even  the  chromosomes  may  appear  perfectly  homogene- 
ous, and  the  same  is  sometimes  true  of  the  entire  nucleus,  as  in  the 
spermatozoon.  The  opinion  is  nevertheless  gaining  ground  that  the 
chromatin-granules  have  a  persistent  identity  and  are  to  be  regarded 
as  morphological  units  of  which  the  chromatin  is  built  up.^ 

Heidenhain  ('93,  '94),  whose  views  have  been  accepted  by  Reinke, 
Waldeyer,  and  others,  has  shown  that  the  "achromatic"  nuclear  net- 
work is  likewise  composed  of  granules  which  he  distinguishes  as 
lantJmnin-  or  oxychromatin-gx2iXvv\^?>  from  the  basic hromatin-gr^r\u\Q?, 
of  the  chromatic  network.  Like  the  latter,  the  oxychromatin-granules 
are  suspended  in  a  non-staining  clear  substance,  for  which  he  reserves 

1  See  Chapter  VII.  -  Cf.  Chapter  VI. 


28  GENERAL   SKETCH  OF   THE    CELL 

the  term  "linin."  Both  forms  of  granules  occur  in  the  chromatic 
network,  while  the  achromatic  network  contains  only  oxychromatin. 
They  are  sharply  differentiated  by  dyes,  the  basichromatin  being- 
coloured  by  the  basic  anilines  (methyl  green,  saffranin,  etc.)  and  other 
true  "nuclear  stains";  while  the  oxychromatin-granules,  like  many 
cytoplasmic  structures,  and  like  the  substance  of  true  nucleoli  (pyrenin), 
are  coloured  by  acid  anilines  (rubin,  eosin,  etc.)  and  other  ''plasma 
stains."  This  distinction,  as  will  appear  in  Chapter  VII.,  is  probably 
one  of  great  physiological  significance. 

Still  other  forms  of  granules  have  been  distinguished  in  the  nucleus 
by  Reinke  ('94)  and  Schloter  ('94).  Of  these  the  most  important 
are  the  "  cedematin-granules,"  which  according  to  the  first  of  these 
authors  form  the  principal  mass  of  the  ground-substance  or  **  nuclear 
sa.p  "  of  Hertwig  and  other  authors.  These  granules  are  identified 
by  both  observers  with  the  *' cyanophilous  granules,"  which  Altmann 
regarded  as  the  essential  elements  of  the  nucleus.  It  is  at  present 
impossible  to  give  a  consistent  interpretation  of  the  morphological 
value  and  physiological  relations  of  these  various  forms  of  granules. 
The  most  that  can  be  said  is  that  the  basichromatin-granules  are 
probably  normal  structures ;  that  they  play  a  principal  role  in  the 
life  of  the  nucleus  ;  that  the  oxychromatin-granules  are  nearly  related 
to  them ;  and  that  not  improbably  the  one  form  may  be  transformed 
into  the  other  in  the  manner  suggested  in  Chapter  VII. 

The  nuclear  membrane  is  not  yet  thoroughly  understood,  and 
much  discussion  has  been  devoted  to  the  question  of  its  origin  and 
structure.  The  most  probable  view  is  that  long  since  advocated  by 
Klein  ('78)  and  Van  Beneden  ('83)  that  the  membrane  arises  as  a 
condensation  of  the  general  protoplasmic  reticulum,  and  is  part  of 
the  same  structure  as  the  linin-network  and  the  cyto-reticulum.  Like 
these,  it  is  in  some  cases  "achromatic,"  but  in  other  cases  it  shows 
the  same  staining  reactions  as  chromatin,  or  may  be  double,  con- 
sisting of  an  outer  achromatic  and  an  inner  chromatic  layer.  Ac- 
cording to  Reinke,  it  consists  of  oxychromatin-granules  like  those  of 
the  linin-network. 


3.    Chemistjy  of  the  Nucleus 

The  chemical  nature  of  the  various  nuclear  elements  will  be  considered  in 
Chapter  VII.,  and  a  brief  statement  will  here  suffice.  The  following  classification 
of  the  nuclear  substances,  proposed  by  Schwarz  in  1887,  has  been  widely  accepted, 
though  open  to  criticism  on  various  grounds. 

1.  Chromatin.     The  chromatic  substance  (basichromatin)  of  the  network  and  of 

those  nucleoli  known  as  net-knots  or  karyosomes. 

2.  Limn.     The  achromatic  network  and  the  spindle-fibres  arising  from  it. 


THE    CYTOPLASM  29 

3.  Paralifnn.     The  ground-substance. 

4.  Fyrenin  or  Parachrojuatin.     The  inner  mass  of  true  nucleoli. 

5.  Aniphipyrenin.     The  substance  of  the  nuclear  membrane. 

Chroinatm  is  probably  identical  with  miclein  (p.  240),  which  is  a  compound  of 
nucleic  acid  (a!' complex  organic  acid,  rich  in  phosphorus)  and  albumin.  In  certain 
cases  (nuclei  of  spermatozoa,  and  probably  also  the  chromosomes  at  the  time  of 
mitosis),  chromatin  may  be  composed  of  nearly  pure  nucleic  acid.  The  linin  is 
probably  composed  of  "plastin,*'  a  substance  similar  to  nuclein,  but  containing  a 
lower  percentage  of  phosphorus,  and  either  belonging  to  the  nucleo-proteids  or 
approaching  them.  It  is  nearly  related  with  the  substance  of  the  cyto-reticulum. 
Pyreniti  consists  of  a  plastin-substance  which  stains  like  linin.  Ajnphipyrefiin  is 
probably  identical  with  linin,  since  the  nuclear  membrane  is  probably  a  condensed 
portion  of  the  general  reticulum  which  forms  the  boundary  between  the  intra-  and 
extra- nuclear  networks.  It  should  be  borne  in  mind,  however,  that  the  membrane 
often  has  an  inner  chromatic  layer  composed  of  chromatin. 


D.     The  Cytoplasm 

It  has  long  been  recognized  that  in  the  unicellular  forms  the 
cytoplasmic  substance  is  often  differentiated  into  two  well-marked 
zones ;  viz.  an  inner  medullary  substance  or  endoplasm  in  which  the 
nucleus  lies,  and  an  outer  cortical  substance  or  exoplasm  (ectoplasm) 
from  which  the  more  differentiated  products  of  the  cytoplasm,  such 
as  cilia,  trichocysts,  and  membrane,  take  their  origin.  Indications  of 
a  similar  differentiation  are  often  shown  in  the  tissue-cells  of  higher 
plants  and  animals,^  though  it  may  take  the  form  of  a  polar  differ- 
entiation of  the  cell-substance,  or  may  be  wholly  wanting.  Whether 
the  distinction  is  of  fundamental  importance  remains  to  be  seen  ;  but 
it  appears  to  be  a  general  rule  that  the  nucleus  is  surrounded  by 
protoplasm  of  relatively  slight  differentiation,  while  the  more  highly 
differentiated  products  of  cell-activity  are  laid  down  in  the  more 
peripheral  region  of  the  cell,  either  in  the  cortical  zone  or  at  one 
end  of  the  cell.^  This 'fact  is  full  of  meaning,  not  only  because  it  is 
an  expression  of  the  adaptation  of  the  cell  to  its  external  environment, 
but  also  because  of  its  bearing  on  the  problems  of  nutrition.^  For  if, 
as  we  shall  see  reason  to  conclude  in  Chapter  VII.,  the  nucleus  be 
immediately  concerned  with  synthetic  metabolism,  we  should  expect 
to  find  the  immediate  and  less  differentiated  products  of  its  action  in 
its  neighbourhood,  and  on  the  whole  the  facts  bear  out  this  view. 

1  This  fact  was  first  pointed  out  in  the  tissue-cells  of  animals  by  Kupffer  ('75),  and  its 
importance  has  since  been  urged  by  Waldeyer,  Reinke,  and  others.  The  cortical  layer  is 
by  Kupffer  termed  paraplasm,  by  Pfeffer  hyaloplasm,  by  Pringsheim  the  Hautschicht.  The 
medullary  zone  is  termed  by  Kupffer,  protoplasm,  sensu  strichi ;  by  Strasburger  Korner- 
plasma,  by  Nageli  polioplasm. 

2  Cf.  p.  38. ' 

3  See  Kupffer  ('90),  pp.  473-476- 


30 


GENERAL   SKETCH  OF   THE    CELL 


The  most  pressing  of  all  questions  regarding  the  cytoplasmic 
structure  is  whether  the  sponge-like,  fibrillar,  or  alveolar  appearance 
is  a  normal  condition  existing  during  life.  There  are  many  cases, 
especially  among  plant-cells,  in  which  the  most  careful  examination 
has  thus  far  failed  to  reveal  the  presence  of  a  reticulum,  the  cyto- 
plasm appearing,  even  under  the  highest  powers  and  after  the  most 


♦  ,.««H»»M«*»<l.«.»«' 


A  C  D 

Fig.  13.  —  Ciliated  cells,  showing  cytoplasmic  fibrillas  terminating  in  a  zone  of  peripheral 
microsomes  to  which  the  cilia  are  attached.     [Engelmann.] 

A.  From  intestinal  epithelium  of  ^«t7flf<?«/a.  B.  From  gill  oi Anodon^a.  CD.  Intestinal  epi- 
thelium of  Cyc/as. 


careful  treatment,  merely  as  a  finely  granular  substance.  This  and 
the  additional  fact  that  the  cytoplasm  may  show  active  streaming  and 
flowing  movements,  has  led  some  authors,  especially  among  bota- 
nists, to  regard  the  reticulum  as  non-essential  and  as  being,  when 
present,  a  secondary  differentiation  of  the  cytoplasmic  substance 
specially  developed  for  the  performance  of  particular  functions.  It 
has  been  shown,  moreover,  that  structureless  proteids,  such  as  egg- 


THE    CYTOFLASM  31 

albumin  and  other  substances,  when  coagulated  by  various  reagents, 
often  show  a  structure  closely  similar  to  that  of  protoplasm  as  ob- 
served in  microscopical  sections.  Biitschli  has  made  careful  studies 
of  such  coagulation-phenomena  which  show  that  coagulated  or  dried 
albumin,  starch-solutions,  gelatin,  gum  arabic,  and  other  substances 
show  a  fine  aveolar  structure  scarcely  to  be  distinguished  from  that 
which  he  believes  to  be  the  normal  and  typical  structure  of  pro- 
toplasm. Fischer  ('94,  '95)  has  made  still  more  extensive  tests  of 
solutions  of  albumin,  peptone,  and  related  substances,  in  various 
degrees  of  concentration,  fixed  and  stained  by  a  great  variety  of  the 
reagents  ordinarily  used  for  the  demonstration  of  cell-structures.  The 
result  was  to  produce  a  marvellously  close  siinulacnim  of  the  appear- 
ances observed  in  the  cell,  reticulated  and  fibrillar  structures  being 
produced  that  often  consist  of  rows  of  granules  closely  similar  in 
every  respect  to  those  described  by  Altmann  and  other  students  of 
the  cell.  After  impregnating  pith  with  peptone-solution  and  then 
hardening,  sectioning,  and  staining,  the  cells  may  even  contain  a 
central  nucleus-like  mass  suspended  in  a  network  of  anastomosing 
threads  that  extend  in  every  direction  outward  to  the  walls,  and 
give  a  remarkable  likeness  of  a  normal  cell. 

These  facts  show  hovCr  cautious  we  must  be  in  judging  the  appear- 
ances seen  in  preserved  cells,  and  justify  in  some  measure  the  hesita- 
tion with  which  many  existing  accounts  of  cell-structure  are  received. 
The  evidence  is  nevertheless  overwhelmingly  strong,  as  I  believe, 
that  not  only  the  fibrillar  and  alveolar  formations,  but  also  the  micro- 
somes observed  in  cell-structures,  are  in  part  normal  structures.  This 
evidence  is  derived  partly  from  a  study  of  the  living  cell,  partly  from 
the  regular  and  characteristic  arrangement  of  the  thread-work  and 
microsomes  in  certain  cases.  In  many  Protozoa,  for  example,  a  fine 
alveolar  structure  may  be  seen  in  the  living  protoplasm  ;  and  Flem- 
ming  as  wxll  as  manyjater  observers  has  clearly  seen  fibrillar  struct- 
ures in  the  living  cells  of  cartilage,  epithelium  connective-tissue, 
and  some  other  animal  cells  (Fig.  9).  Mikosch,  als6,  has  recently 
described  granular  threads  in  living  plant-cells. 

Almost  equally  conclusive  is  the  beautifully  regular  arrangement 
of  the  fibrillce  in  ciliated  cells  (Fig.  13,  Engelmann),  in  muscle-fibres 
and  nerve  fibres,  and  especially  in  the  mitotic  figure  of  dividing-cells 
(Figs.  16,  24),  where  they  are  likewise  more  or  less  clearly  visible 
in  life.  A  very  convincing  case  is  afforded  by  the  pancreas-cells 
of  Nccturus,  which  Mathews  has  carefully  studied  in  my  laboratory. 
Here  the  thread-work  consists  of  long,  conspicuous,  definite  fibrillae, 
some  of  which  may  under  certain  conditions  be  wound  up  more  or 
less  clearly  in  a  spiral  mass  to  form  the  so-called  Xcbciikcni.  In  all 
these  cases  it  is  impossible  to  regard  the  thread- work  as  an  accidental 


32 


GENERAL    SKETCH   OF   THE    CELL 


coagulation-product.  On  the  whole,  therefore,  it  is  probable  that 
careful  treatment  by  reagents  gives  at  least  an  approximately  true 
picture  of  the  normal  thread-work,  though  we  must  always  allow  for 
the  possible  occurrence  of  artificial  products. 


iSSv 


,«5^^^%. 

,. 

% 

f'-:-'-     '            '■:' 

1 

Hi 

Fig.  14.  —  Section  through  a  nephridial  cell  of  the  leech,  Clepsine  (drawn  by  Arnold  Graf  from 
one  of  his  own  preparations). 

The  centre  of  the  cell  is  occupied  by  a  large  vacuole,  filled  with  a  watery  liquid.  The  cyto- 
plasm forms  a  very  regular  and  distinct  reticulum  with  scattered  microsomes  which  become  very 
large  in  the  peripheral  zone.  The  larger  pale  bodies,  lying  in  the  groimd-substance,  are  excretory 
granules  (f>.  metaplasm).  The  nucleus,  at  the  right,  is  surrounded  by  a  thick  chromatic  mem- 
brane, is  traversed  by  a  very  distinct  linin-network,  contains  numerous  scattered  chromatin- 
granules,  and  a  single  large  nucleolus  within  which  is  a  vacuole.  Above  are  two  isolated  nuclei 
showing  nucleoli  and  chromatin-granules  suspended  on  the  linin-threads. 

One  of  the  most  beautiful  forms  of  cyto-reticulum  with  which  I 
am  acquainted  has  been  described  by  Bolsius  and  Graf  in  the  ne- 


THE    CYTOPLASM 


33 


phridial  cells  of  leeches  as  shown  in  Fig.  14  (from  a  preparation  by 
Dr.  Arnold  Graf).  The  reticulum  is  here  of  great  distinctness  and 
regularity,  and  scattered  microsomes  are  found  along  its  threads.  It 
appears  with  equal  clearness,  though  in  a  somewhat  different  form, 


t 


Fig.  15.  —  Spinal  ganglion-cell  of  the  frog.  [VON  LenhosSEK.] 
The  nucleus  contains  a  single  intensely  chromatic  nucleolus,  and  a  paler  linin-network  with 
rounded  chromatin-granules.  The  cytoplasmic  fibrillae  are  faintly  shown  passing  out  into  the 
nerve-process  below.  (They  are  figured  as  far  more  distinct  by  Flemming.)  The  dark  cyto- 
plasmic masses  are  the  deeply  staining  "  chromophilic  granules"  (Nissl)  of  unknown  function. 
(The  centrosome,  which  lies  near  the  centre  of  the  cell,  is  shown  in  Fig.  7,  C)  At  the  left,  two 
connective  tissue-cells. 

in  many  eggs,  where  the  meshes  are  rounded  and  often  contain  food- 
matters  or  deutoplasm  in  the  inter-spaces  (Figs.  42,  43).  In  cartilage- 
cells  and  connective  tissue-cells,  where  the  threads  can  be  plainly  seen 
in  life,  the  network  is  loose  and  open,  and  appears  to  consist  of  more 
or  less  completely  separate  threads  (Fig.  9).     In  the  cells  of  colum- 

D 


34  GENERAL   SKETCH   OF   THE    CELL 

nar  epithelium,  the  threads  in  the  peripheral  part  of  the  cell  often 
assume  a  more  or  less  parallel  course,  passing  outwards  from  the 
central  region,  and  giving  the  outer  zone  of  the  cell  a  striated  appear- 
ance. This  is  very  conspicuously  shown  in  ciliated  epithelium,  the 
fibrillae  corresponding  in  number  with  the  cilia  as  if  continuous  with 
their  bases  (Fig.  13).^  In  nerve-fibres  the  threads  form  closely  set 
parallel  fibrillae  which  may  be  traced  into  the  body  of  the  nerve-cell ; 
here,  according  to  most  authors,  they  break  up  into  a  network  in 
which  are  suspended  numerous  deeply  staining  masses,  the  "  chromo- 
philic  granules"  of  Nissl  (Fig.  15).  In  the  contractile  tissues  the 
threads  are  in  most  cases  very  conspicuous  and  have  a  parallel  course. 
This  is  clearly  shown  in  smooth  muscle-fibres  and  also,  as  Ballowitz 
has  shown,  in  the  tails  of  spermatozoa.  This  arrangement  is  most 
striking  in  striped  muscle-fibres  where  the  fibrillae  are  extremely  well 
marked.  According  to  Retzius,  Carnoy,  Van  Gehuchten,  and  others, 
the  meshes  have  here  a  rectangular  form,  the  principal  fibrillae  having 
a  longitudinal  course  and  being  connected  at  regular  intervals  by 
transverse  threads ;  but  the  structure  of  the  muscle-fibre  is  probably 
far  more  complicated  than  this  account  would  lead  one  to  suppose, 
and  opinion  is  still  divided  as  to  whether  the  contractile  substance 
is  represented  by  the  reticulum  proper  or  by  the  ground-substance. 

Nowhere,  however,  is  the  thread-work  shown  with  such  beauty 
as  in  dividing-cells,  where  (Figs.  16,  24)  the  fibrillae  group  themselves 
in  two  radiating  systems  or  asters^  which  are  in  some  manner  the 
immediate  agents  of  cell-division.  Similar  radiating  systems  of  fibres 
occur  in  amoeboid  cells,  such  as  leucocytes  (Fig.  35)  and  pigment- 
cells  (Fig.  36),  where  they  probably  form  a  contractile  system  by 
means  of  which  the  movements  of  the  cell  are  performed. 

The  views  of  Butschli  and  his  followers,  which  have  been  touched 
on  at  p.  18,  differ  considerably  from  the  foregoing,  the  fibrillae 
being  regarded  as  the  optical  sections  of  thin  plates  or  lamellae 
which  form  the  walls  of  closed  chambers  filled  by  a  more  liquid 
substance.  Butschli,  followed  by  Reinke,  Eismond,  Erlanger,  and 
others,  interprets  in  the  same  sense  the  astral  systems  of  dividing- 
cells  which  are  regarded  as  a  radial  configuration  of  the  lamellae 
about  a  central  point  (Fig.  8,  B\  Strong  evidence  against  this  view 
is,  I  believe,  afforded  by  the  appearance  of  the  spindle  and  asters 
in  cross-section.  In  the  early  stages  of  the  egg  of  Nereis,  for 
example,  the  astral  rays  are  coarse  anastomosing  fibres  that  stain 
intensely  and  are  therefore  very  favourable  for  observation  (Fig.  43). 
That  they  are  actual  fibres  is,  I  think,  proved  by  sagittal  sections 
of  the   asters  in   which   the   rays   are   cut   at  various   angles.     The 

1  The  structure  of  the  ciliated  cell,  as  described  by  Engelmann,  may  be  beautifully 
demonstrated  in  the  funnel-cells  of  the  nephridia  and  sperm-ducts  of  the  earthworm. 


THE    CYTOPLASM 


35 


cut  ends  of  the  branching  rays  appear  in  the  clearest  manner,  not 
as  plates  but  as  distinct  dots,  from  which  in  oblique  sections  the 
ray  may  be  traced  inwards  towards  the  centrosphere.  Driiner,  too, 
figures  the  spindle  in  cross-section  as  consisting  of  rounded  dots, 
like  the  end  of  a  bundle  of  wires,  though  these  are  connected  by 
cross-branches  (Fig.  22,  F\  Again,  the  crossing  of  the  rays  pro- 
ceeding from  the  asters  (Fig.  69),  and  their  behaviour  in  certain 
phases  of  cell-division,  is  difficult  to  explain  under  any  other  than 
the  fibrillar  theory. 

We    must   admit,    however,    that   the    network   varies    greatly    in 


Centrosphere  con- 
taining the  cen- 
trosome. 


Aster. 


Spindle. 


Chromosomes  forming  the  equatorial  plate. 

Fig.  16.  —  Diagram  of  the  dividing  cell,  showing  the  mitotic  figure  and  its  relation  to  the  cyto- 
reticulum. 

different  cells  and  even  in  different  physiological  phases  of  the 
same  cell;  and  that  it  is  impossible  at  present  to  bring  it  under 
any  rule  of  universal  application.  It  is  possible,  nay  probable,  that 
in  one  and  the  same  cell  a  portion  of  the  network  may  form  a 
true  alveolar  structure  such  as  is  described  by  Biitschli,  while  other 
portions  may,  at  the  same  time,  be  differentiated  into  actual  fibres. 
If  this  be  true  the  fibrillar  or  alveolar  structure  is  a  matter  of 
secondary  moment,  and  the  essential  features  of  protoplasmic  organ- 
ization must  be  sought  in  a  more  subtle  underlying  structure. ^ 


^  See  Chapter  VI. 


36  GENERAL   SKETCH   OF   THE    CELL 


E.     The  Centrosome 

No  element  of  the  cell  has  aroused  a  wider  interest  of  late  than 
the  remarkable  body  known  as  the  cent7'osoinc,  which  is  now  gener- 
ally regarded  as  the  especial  organ  of  cell-division,  and  in  this  sense 
as  the  dynamic  cejitre  of  the  cell  (Van  Beneden,  Boveriy  In  its 
simplest  form  the  centrosome  is  a  body  of  extreme  minuteness,  often 
indeed  scarce  larger  than  a  microsome,  which  nevertheless  exerts 
an  extraordinary  influence  on  the  cytoplasmic  network  during  cell- 
division  and  the  fertilization  of  the  ^g^.  As  a  rule  it  lies  out- 
side, though  near,  the  nucleus,  in  the  cyto-reticulum,  surrounded 
by  a  granular,  reticular,  or  radiating  area  of  the  latter  known 
as  the  attraction-sphere  or  centrospJiere  (Figs.  5,  6,  j)?  It  may, 
however,  lie  within  the  nuclear  membrane  in  the  linin-network 
(Fig.  107).  In  some  cases  the  centrosome  is  a  single  body  which 
divides  into  two  as  the  cell  prepares  for  division.  More  commonly, 
it  becomes  double  during  the  later  phases  of  cell-division,  in  anticipa- 
tion of  the  succeeding  division,  the  two  centrosomes  thus  formed 
lying  passively  within  the  attraction-sphere  during  the  ordinary  life 
of  the  cell.  They  only  become  active  as  the  cell  prepares  for  the 
ensuing  division,  when  they  diverge  from  one  another,  and  each 
becomes  the  centre  of  one  of  the  astral  systems  referred  to  at 
p.  49.  Each  of  the  daughter-cells  receives  one  of  the  centrosomes, 
which  meanwhile  again  divide  into  two.  The  centrosome  seems, 
therefore,  to  be  in  some  cases  a  permanent  cell-organ,  like  the 
nucleus,  being  handed  on  by  division  from  one  cell  to  another. 
There  are,  however,  some  cells,  e.g.  muscle-cells,  most  gland-cells, 
and  many  unicellular  organisms,  in  which  no  centrosome  has  thus 
far  been  discovered  in  the  resting-cell ;  but  it  is  uncertain  whether 
the  centrosome  is  really  absent  in  such  cases,  for  it  may  be  hidden 
in  the  nucleus,  or  too  small  to  be  distinguished  from  other  bodies 
in  the  cytoplasm.  There  is,  however,  good  reason  to  believe  that 
it  degenerates  and  disappears  in  the  mature  eggs  of  many  animals, 
and  this  may  likewise  occur  in  other  cells.  At  present,  therefore, 
we  are  not  able  to  say  whether  the  centrosome  is  of  equal  constancy 
with  the  nucleus.^ 

1  The  centrosome  was  discovered  by  Van  Beneden  in  the  cells  of  Dycyemids  ('76),  and 
first  carefully  described  by  him  in  the  egg  of  Ascaris  seven  years  later.  The  name  is  due 
to  Boveri  ('88,  2,  p.  68). 

'^  Cf.  p.  229. 

2  Its  nature  is  more  fully  discussed  at  p.  224. 


OTHER    ORGANS  37 


F.     Other  Organs 

The  cell-substance  is  often  differentiated  into  other  more  or  less 
definite  Structures,  sometimes  of  a  transitory  character,  sometimes 
showing  a  constancy  and  morphological  persistency  comparable  with 
that  of  the  nucleus  and  centrosome.  From  a  general  point  of  view 
the  most  interesting  of  these  are  the  bodies  known  as  plastids  or  proto- 
plasts (¥\g.  5),  which,  like  the  nucleus  and  centrosome,  are  capable  of 
growth  and  division,  and  may  thus  be  handed  on  from  cell  to  cell. 
The  most  important  of  these  are  the  chromatophores  or  chromoplasts, 
which  are  especially  characteristic  of  plants,  though  they  occur  in 
some  animals  as  well.  These-  are  definite  bodies,  varying  greatly  in 
form  and  size,  which  never  arise  spontaneously,  so  far  as  known,  but 
always  by  the  division  of  pre-existing  bodies  of  the  same  kind.  They 
possess  in  some  cases  a  high  degree  of  morphological  independence, 
and  may  even  live  for  a  time  after  removal  from  the  remaining  cell- 
substance,  as  in  the  case  of  the  "yellow  cells"  of  Radiolaria.  This 
has  led  to  the  view,  advocated  by  Brandt  and  others,  that  the 
chlorophyll-bodies  found  in  the  cells  of  many  Protozoa  and  a  few 
Metazoa  {Hydra,  Spongilla,  some  Planarians)  are  in  reality  distinct 
Algae  living  symbiotically  in  the  cell.  This  view  is  probably  correct 
in  some  cases,  e.g.  in  the  Radiolaria ;  but  it  may  well  be  doubted 
whether  it  is  of  general  application.  In  the  plants  the  chlorophyll- 
bodies  and  other  chromoplasts  are  almost  certainly  to  be  regarded  as 
differentiations  of  the  cytoplasmic  substance.  The  same  is  true  of 
the  amyloplasts,  which  act  as  centres  for  the  formation  of  starch. 

The  contractile  or  pulsating  vacuoles  that  occur  in  most  Protozoa 
and  in  the  swarm-spores  of  many  Algae  are  also  known  in  some 
cases  to  multiply  by  division  ;  and  the  same  is  true,  according  to  the 
researches  of  De  Vries,  Went,  and  others,  of  the  non-pulsating  vacu- 
oles of  plant-cells.  These  vacuoles  have  been  shown  to  have,  in  many 
cases,  distinct  walls,  and  they  are  regarded  by  De  Vries  as  a  special 
form  of  plastid  ("tonoplasts")  analogous  to  the  chromatophores  and 
other  plastids.  It  is,  however,  probable  that  this  view  is  only  appli- 
cable to  certain  forms  of  vacuoles. 

The  existence  of  cell-organs  which  have  the  power  of  independent 
assimilation,  growth,  and  division,  is  a  fact  of  great  theoretical  interest 
in  its  bearing  on  the  general  problem  of  cell-organization  ;  for  it  is 
one  of  the  main  reasons  that  have  led  De  Vries,  Wiesner,  and  many 
others  to  regard  the  entire  cell  as  made  up  of  elementary  self- 
propagating  units. 


38  GENERAL   SKETCH  OF   THE    CELL 


G.     The  Cell-membrane 

From  a  general  point  of  view  the  cell-membrane  or  intercellular 
substance  is  of  relatively  minor  importance,  since  it  is  not  of  constant 
occurrence,  belongs  to  the  lifeless  products  of  the  cell,  and  hence 
plays  no  direct  part  in  the  active  cell-life.  In  plant-tissues  the  mem- 
brane is  almost  invariably  present  and  of  firm  consistency.  Animal 
tissues  are  in  general  characterized  by  the  slight  development  or 
absence  of  cell-walls.  Many  forms  of  cells,  both  among  unicellular 
and  multicellular  forms,  are  quite  naked,  for  example  Aviceba  and  the 
leucocytes ;  but  in  most,  if  not  in  all,  such  cases,  the  outer  limit  of 
the  cell-body  is  formed  by  a  more  resistant  layer  of  protoplasm  —  the 
"pellicle"  of  Biitschli  —  that  may  be  so  marked  as  to  simulate  a  true 
membrane,  for  example,  in  the  red  blood-corpuscles  (Ranvier,  Wal- 
deyer)  and  in  various  naked  animal  eggs.  Such  a  ''  pellicle  "  differs 
from  a  true  cell-membrane  only  in  degree ;  and  it  is  now  generally 
agreed  that  the  membranes  of  plant-cells,  and  of  many  animal-cells, 
arise  by  a  direct  physical  and  chemical  transformation  of  the  periph- 
eral layer  of  protoplasm.  On  the  other  hand,  according  to  Leydig, 
Waldeyer,  and  some  others,  the  membrane  of  certain  animal-cells  may 
be  formed  not  by  a  direct  transformation  of  the  protoplasmic  substance, 
but  as  a  secretion  poured  out  by  the  protoplasm  at  its  surface.  Such 
membranes,  characterized  as  "  cuticular,"  occur  mainly  or  exclusively 
on  the  free  surfaces  of  cells  (Waldeyer).  It  remains  to  be  seen,  how- 
ever, how  far  this  distinction  can  be  maintained,  and  the  greatest 
diversity  of  opinion  still  exists  regarding  the  origin  of  the  different 
forms  of  cell-membranes  in  animal-cells. 

The  chemical  composition  of  the  membrane  or  intercellular  sub- 
stance varies  extremely.  In  plants  membrane  consists  of  a  basis  of 
cellulose,  a  carbohydrate  having  the  formula  CgHjoOg ;  but  this  sub- 
stance is  very  frequently  impregnated  with  other  substances,  such 
as  silica,  lignin,  and  a  great  variety  of  others.  In  animals  the  inter- 
cellular substances  show  a  still  greater  diversity.  Many  of  them  are 
nitrogenous  bodies,  such  as  keratin,  chitin,  elastin,  gelatin,  and  the 
like ;  but  inorganic  deposits,  such  as  silica  and  carbonate  of  lime,  are 
common. 

H.     Polarity  of  the  Cell 

In  a  large  number  of  cases  the  cell  exhibits  a  definite  polarity,  its 
parts  being  symmetrically  grouped  with  reference  to  an  ideal  organic 
axis  passing  from  pole  to  pole.  No  definite  criterion  for  the  identi- 
fication of  the  cell-axis  has,  however,  yet  been  determined;  for  the 


POLARITY   OF   THE    CELL 


39 


general  conception  of  cell-polarity  has  been  developed  in  two  differ- 
ent directions,  one  of  which  starts  from  purely  morphological  con- 
siderations, the  other  from  physiological,  and  a  parallelism  between 
them  has  not  thus  far  been  very  clearly  made  out. 

On  the^one  hand.  Van  Beneden  ('83)  conceived  cell-polarity  as  a 
primary  morphological  attribute  of  the  cell,  the  organic  axis  being 
identified  as  a  line  drawn  through  the  centre  of  the  nucleus  and  the 
centrosome  (Fig.  17,  A).  With  this  view  Rabl's  theory  ('85)  of 
nuclear  polarity  harmonizes,  for  the  chromosome-loops  converge  tow- 
ards the  centrosome,  and  the  nuclear  axis  coincides  with  the  cell-axis. 
Moreover,  it  identifies  the  polarity  of  the  ^^g,  which  is  so  important 
a  factor  in  development,  with  that  of  the  tissue-cells;  for  the  ^gg- 


A 

Van  Beneden. 


B  C 

Rabl,  Hatschek. 


Fig.  17.  —  Diagrams  of  cell-polarity. 
A.  Morphological  polarity  of  Van  Beneden.    Axis  passing  through  nucleus  and  centrosome. 
Chromatin-threads    converging   towards  the  centrosome.     B.  C.    Physiological  polarity  of  Rabl 
and  Hatschek,  ^  in  a  gland-ceH,  C  in  a  ciliated  cell. 


centrosome  almost   invariably  appears    at  or   near  one  pole  of   the 
ovum. 

Heidenhain  ('94,  '95)  has  recently  developed  this  conception  of 
polarity  in  a  very  elaborate  manner,  maintaining  that  all  the  struct- 
ures of  the  cell  have  a  definite  relation  to  the  primary  axis,  and  that 
this  relation  is  determined  by  conditions  of  tension  in  the  astral  rays 
focussed  at  the  centrosome.  On  this  basis  he  endeavours  to  explain 
the  position  and  movements  of  the  nucleus,  the  succession  of  division- 
planes,  and  many  related  phenomena.  In  the  present  state  of  the 
subject,  Heidenhain's  theories  must  be  regarded  as  somewhat  trans- 
cendental, though  they  give  many  suggestions  for  further  investigation. 


40  GENERAL   SKETCH   OF   THE    CELL 

Hatschek  ('88)  and  Rabl  ('89,  '92),  on  the  other  hand,  have  ad- 
vanced a  quite  different  hypothesis  based  on  physiological  considera- 
tions. By  **  cell-polarity  "  these  authors  mean,  not  a  predetermined 
morphological  arrangement  of  parts  in  the  cell,  but  a  polar  differen- 
tiation of  the  cell-substance  arising  secondarily  through  adaptation  of 
the  cell  to  its  environment  in  the  tissues,  and  having  no  necessary 
relation  to  the  polarity  of  Van  Beneden.  (Fig.  17,  B,  C.)  This  is 
typically  shown  in  epithelium,  which,  as  Kolliker  and  Hackel  long 
since  pointed  out,  is  to  be  regarded,  both  ontogenetically  and  phy- 
logenetically,  as  the  most  primitive  form  of  tissue.  The  free  and 
basal  ends  of  the  cells  here  differ  widely  in  relation  to  the  food- 
supply,  and  show  a  corresponding  structural  differentiation.  In  such 
cells  the  nucleus  usually  lies  nearer  the  basal  end,  towards  the  source 
of  food,  while  differentiated  products  of  the  cell-activity  are  formed 
either  at  the  free  end  (cuticular  structures,  cilia,  pigment,  zymogen- 
granules),  or  at  the  basal  end  (muscle-fibres,  nerve-fibres).  In  the 
non-epithelial  tissues  the  polarity  may  be  lost,  though  traces  of  it 
are  often  shown  as  a  survival  of  the  epithelial  arrangement  of  the 
embryonic  stages. 

But,  although  this  conception  of  polarity  has  an  entirely  different 
point  of  departure  from  Van  Heneden's,  it  leads,  in  some  cases  at 
least,  to  the  same  result ;  for  the  cell-axis,  as  thus  determined,  may 
coincide  with  the  morphological  axis  as  determined  by  the  position 
of  the  centrosome.  This  is  the  case,  for  example,  with  both  the 
spermatozoon  and  the  ovum ;  for  the  morphological  axis  in  both  is 
also  the  physiological  axis  about  which  the  cytoplasmic  differentiations 
are  grouped.  Moreover,  the  observations  of  Heidenhain,  Lebrun,  and 
Kostanecki  indicate  that  the  same  is  true  in  epithelium ;  for,  accord- 
ing to  these  authors,  the  centrosome  is  always  situated  on  that  side 
of  the  nucleus  turned  towards  the  free  end  of  the  cell.  How  far  this 
law  holds  good  remains  to  be  seen,  and,  until  the  facts  have  been 
further  investigated,  it  is  impossible  to  frame  a  consistent  hypothesis 
of  cell-polarity.  The  facts  observed  in  epithelial  cells,  are,  however, 
of  great  significance  ;  for  the  position  of  the  centrosome,  and  hence 
the  direction  of  the  axis,  is  here  obviously  related  to  the  cell-environ- 
ment, and  it  is  difficult  to  avoid  the  conclusion  that  the  latter  must 
be  the  determining  condition  to  which  the  intracellular  relations  con- 
form. When  applied  to  the  germ-cells,  this  conclusion  becomes  of 
high  interest ;  for  the  polarity  of  the  egg  is  one  of  the  primary  con- 
ditions of  development,  and  we  have  here,  as  I  believe,  a  clue  to  its 
origin. 1 

I  Cf.  pp.  288,  320. 


THE    CELL   IN  RELATION   TO    THE  MULTICELLULAR  BODY      4 1 


I.     The  Cell  in  Relation  to  the  Multicellular  Body 

In  analyzing  the  structure  and  functions  of  the  individual  cell  we 
are  accustomed,  as  a  matter  of  convenience,  to  regard  it  as  an  inde- 
pendent elementary  organism  or  organic  unit.  Actually,  however, 
it  is  such  an  organism  only  in  the  case  of  the  unicellular  plants  and 
animals  and  the  germ-cells  of  the  multicellular  forms.  When  we 
consider  the  tissue-cells  of  the  latter  we  must  take  a  somewhat  dif- 
ferent view.  As  far  as  structure  and  origin  are  concerned  the  tissue- 
cell  is  unquestionably  of  the  same  morphological  value  as  the 
one-celled  plant  or  animal ;  and  in  this  sense  the  multicellular  body 
is  equivalent  to  a  colony  or  aggregate  of  one-celled  forms.  Physi- 
ologically, however,  the  tissue-cell  can  only  in  a  limited  sense  be 
regarded  as  an  independent  unit;  for  its  autonomy  is  merged  in  a 
greater  or  less  degree  into  the  general  life  of  the  organism.  From 
this  point  of  view  the  tissue-cell  must  in  fact  be  treated  as  merely  a 
localized  area  of  activity,  provided  it  is  true  with  the  complete  appa- 
ratus of  cell-life,  and  even  capable  of  independent  action  within 
certain  limits,  yet  nevertheless  a  part  and  not  a  whole. 

There  is  at  present  no  biological  question  of  greater  moment  than 
the  means  by  which  the  individual  cell-activities  are  co-ordinated,  and 
the  organic  unity  of  the  body  maintained ;  for  upon  this  question 
hangs  not  only  the  problem  of  the  transmission  of  acquired  charac- 
ters, and  the  nature  of  development,  but  our  conception  of  life  itself. 
Schwann,  the  father  of  the  cell-theory,  very  clearly  perceived  this ; 
and  after  an  admirably  lucid  discussion  of  the  facts  known  to  him 
(1839),  drew  the  conclusion  that  the  life  of  the  organism  is  essentially 
a  composite ;  that  each  cell  has  its  independent  life ;  and  that  "  the 
whole  organism  subsists  only  by  means  of  the  reciprocal  action  of  the 
single  elementary  parts.  "^  This  conclusion,  afterwards  elaborated  by 
Virchow  and  Hackel  to  the  theory  of  the  *' cell-state,"  took  a  very 
strong  hold  on  the  minds  of  biological  investigators,  and  is  even  now 
widely  accepted.  It  is,  however,  becoming  more  and  more  clearly 
apparent  that  this  conception  expresses  only  a  part  of  the  truth,  and 
that  Schwann  went  too  far  in  denying  the  influence  of  the  totality  of 
the  organism  upon  the  local  activities  of  the  cells.  It  would  of 
course  be  absurd  to  maintain  that  the  whole  can  consist  of  more  than 
the  sum  of  its  parts.  Yet,  as  far  as  growth  and  development  are  con- 
cerned, it  has  now  been  clearly  demonstrated  that  only  in  a  limited 
sense  can  the  cells  be  regarded  as  co-operating  units.  They  are 
rather   local   centres  of    a  formative  power  pervading  the  growing 

1  Unio'suchungen,  p.  191. 


42  GENERAL   SKETCH   OF   THE    CELL 

mass  as  a  whole,^  and  the  physiological  autonomy  of  the  individual 
cell  falls  into  the  background.  It  is  true  that  the  cells  may  acquire 
a  high  degree  of  physiological  independence  in  the  later  stages  of 
embryological  development.  The  facts  to  be  discussed  in  the  eighth 
and  ninth  chapters  will,  however,  show  strong  reason  for  the  conclu- 
sion that  this  is  a  secondary  result  of  development  through  which  the 
cells  become,  as  it  were,  emancipated  in  a  greater  or  less  degree, 
from  the  general  control.  Broadly  viewed,  therefore,  the  life  of  the 
multicellular  organism  is  to  be  conceived  as  a  whole ;  and  the  appar- 
ently composite  character,  which  it  may  exhibit,  is  owing  to  a  second- 
ary distribution  of  its  energies  among  local  centres  of  action."^ 

In  this  light  the  structural  relations  of  tissue-cells  becomes  a  ques- 
tion of  great  interest ;  for  we  have  here  to  seek  the  means  by  which 
the  individual  cell  comes  into  relation  with  the  totality  of  the  organ- 
ism, and  by  which  the  general  equilibrium  of  the  body  is  maintained. 
It  must  be  confessed  that  the  results  of  microscopical  research  have 
not  thus  far  given  a  very  certain  answer  to  this  question.  Though 
the  tissue-cells  are  often  apparently  separated  from  one  another  by  a 
non-living  intercellular  substance,  which  may  appear  in  the  form  of 
solid  walls,  it  is  by  no  means  certain  that  their  organic  continuity  is 
thus  actually  severed.  Many  cases  are  known  in  which  division  of 
the  nucleus  is  not  followed  by  division  of  the  cell-body,  so  that  multi- 
nuclear  cells  or  syncytia  are  thus  formed,  consisting  of  a  continuous 
mass  of  protoplasm  through  which  the  nuclei  are  scattered.  Heitz- 
mann  long  since  contended  ('73),  though  on  insufficient  evidence,  that 
division  is  incomplete  in  nearly  all  forms  of  tissue,  and  that  even  when 
cell-walls  are  formed  they  are  traversed  by  strands  of  protoplasm  by 
means  of  which  the  cell-bodies  remain  in  organic  continuity.  The 
whole  body  was  thus  conceived  by  him  as  a  syncytium,  the  cells 
being  no  more  than  nodal  points  in  a  general  reticulum,  and  the  body 
forming  a  continuous  protoplasmic  mass. 

This  interesting  view,  long  received  with  scepticism,  has  been  in  a 
measure  sustained  by  later  researches,  though  it  still  remains  stib 
judice.  Tangl,  Gardiner,  and  many  later  observers  have  shown  that 
the  cell-walls  of  many  plant-tissues  are  traversed  by  delicate  intercel- 
lular bridges,  and  similar  bridges  have  been  conclusively  demon- 
strated by  Bizzozero,  Retzius,  Flemming,  Pfitzner,  and  many  others 
in  the  case  of  animal  epithelial  cells  (Figs,  i,  9).  The  same  has 
been  asserted  to  be  the  case  with  the  smooth  muscle-fibres,  with  car- 
tilage-cells and  connective-tissue  cells,  and  in  a  few  cases  with  nerve- 
cells.  Paladino  and  Retzius  ('89)  have  endeavoured  to  show,  further, 
that  the  follicle-cells   of   the   ovary  are   connected   by   protoplasmic 

iCf.  Chapters  VIII.,  IX. 

2  For  a  fuller  discussion  see  pp.  293  and  311. 


THE    CELL   IN  RELATION   10    THE  MULTICELLULAR  BODY       43 

bridges  not  only  with  one  another,  bitt  also  zvitJi  tlie  ozmvi,  a  conclu- 
sion which,  if  established  by  further  research,  will  be  of  the  greatest 
interest. 

As  far  as  adult  animal-tissues  are  concerned,  it  still  remains  unde- 
termined how  far  the  cells  are  in  direct  protoplasmic  continuity.  It 
is  obvious  that  no  such  continuity  exists  in  the  case  of  the  corpuscles 
of  blood  and  lymph  and  the  wandering  leucocytes  and  pigment-cells. 
In  case  of  the  nervous  system,  which  from  an  a  priori  point  of  view 
would  seem  to  be  above  all  others  the  structure  in  which  protoplasmic 
continuity  is  to  be  expected,  the  latest  researches  are  rendering  it 
more  and  more  probable  that  no  such  continuity  exists,  and  that 
nerve-impulses  are  transmitted  from  cell  to  cell  by  contact-action. 
When,  however,  we  turn  to  the  embryonic  stages  we  find  strong 
reason  for  the  belief  that  a  material  continuity  between  cells  must 
exist.  This  is  certainly  the  case  in  the  early  stages  of  many  arthro- 
pods, where  the  whole  embryo  is  at  first  an  unmistakable  syncytium  ; 
and  Adam  Sedgwick  has  endeavoured  to  show  that  in  Peripatiis  and 
even  in  the  vertebrates  the  entire  embryonic  body,  up  to  a  late  stage, 
is  a  continuous  syncytium.  I  have  pointed  out  ('93)  that  even  in  a 
total  cleavage,  such  as  that  of  Amphioxjis  or  the  echinoderms,  the 
results  of  experiment  on  the  early  stages  of  cleavage  are  difficult  to 
explain,  save  under  the  assumption  that  there  must  be  a  structural 
continuity  from  cell  to  cell  that  is  broken  by  mechanical  displacement 
of  the  blastomeres.  This  conclusion  is  supported  by  the  recent  work 
of  Hammar  ('96),  whose  observations  on  sea-urchin  eggs  I  can  in  the 
main  confirm. 

As  the  subject  now  lies,  however,  the  facts  do  not,  I  believe,  jus- 
tify any  general  statement  regarding  the  occurrence,  origin,  or  physi- 
ological meaning  of  the  protoplasmic  continuity  of  cells ;  and  a  most 
important  field  here  lies  open  for  future  investigation. 


LITERATURE.     P 

Altmann,  R.  —  Die  Elementarorganismen  und  ihre  Beziehungen  zu  den  Zellen,  2d 

ed.     Leipzig,  1894. 
Van  Beneden,  E.  —  (See  Lists  IL,  IV.) 
Boveri,  Th.  —  (See  Lists  IV.,  V.) 
Butschli,  0.  —  Untersuchungen  iiber  mikroskopische  Schaume  und  das  Protoplasma. 

Leipzig  (Engelmann),  1892. 
Engelmann,  T.  W.  —  Zur  Anatomie  und  Physiologie  der  Flimmerzellen  :  Arch.  ges. 

Phys.,  XXIII.     1880. 
von  Erlanger,  R.  —  Neuere  Ansichten  Uber  die  Struktur  des  Protoplasmas :  ZooL 

CeniraldL,  in.  S,g.     1896. 

1  See  also  Introductory  list,  p.  12. 


44  GENERAL   SKETCH  OF   THE    CELL 

Flemming,  W.  —  Zellsubstanz,  Kern  und  Zellteilung.     Leipzig^  1882. 

Id.  —  Zelle  :  Merkel  und  Bonnefs  Ergebnisse,  I .-IV.     1891-94.    (Admirable  reviews 

and  literature-lists.) 
Heidenhain,  M.  —  Uber   Kern   und   Protoplasma :  Festschr.  z.  $o-jd/ir.  Doctorjub. 

von  V.  Kolliker.     Leipzig,  1893. 
Klein,  E.  —  Observations  on  the  Structure  of  Cells  and  Nuclei :   Quart.  Journ.  Mic. 

Sci.,  XVIII.     1878. 
Kolliker,  A.  —  Handbuch  der  Gewebelehre,  6th  ed.     Leipzig,  1889. 
Leydig,  Fr.  —  Zelle  und  Gewebe.     Bonn,  1885. 
Schafer,  E.  A.  —  General  Anatomy  or  Histology;   in  Quain'^s  Anatomy,  I.  2,  loth 

ed.     London,  1891. 
Schiefferdecker  &  Kossel.  —  Die  Gewebe  des  Menschlichen  Korpers.    Braunschweig, 

1891. 
Schwarz,  Fr.  —  Die  morphologische  und  chemische  Zusammensetzung  des  Proto- 

plasmas.     Breslaii,  1887. 
Strasburger,  E.  —  Zellbildung  und  Zellteilung,  3d  ed.     1880. 
Strieker,  S.  —  Handbuch  der  Lehre  von  den  Geweben.     Leipzig,  1871. 
Thoma,  R.  —  Text-book  of  General  Pathology  and  Pathological  Anatomy  :  trans,  by 

Alex.  Bruce.     London,  1896. 
De  Vries,  H.  —  Intracellulare  Pangenesis.     Jena,  1889. 
Waldeyer,  W.  —  Die  neueren  Ansichten  iiber  den  Bau  und  das  Wesen  der  Zelle: 

Deiitsch.  Med.  IVochenschr.,  Oct.,  Nov.,  1895. 
Wiesner,  J.  —  Die   Elementarstruktur  u.   das  Wachstum  der   lebenden   Substanz : 

Wien,  Holder.     1892. 
Zimmerman,   A.  —  Beitrage    zur   Morphologic   und    Physiologic    der   Pflanzenzelle. 

Tubingen,  1893. 


CHAPTER    II 

CELL-DIVISION 

"  Wo  eine  Zelle  entsteht,  da  muss  eine  Zelle  vorausgegangen  sein,  ebenso  wie  das  Thier 
nur  aus  dem  Thiere,  die  Pflanze  nur  aus  der  Pflanze  entstehen  kann.  Auf  diese  Weise  ist, 
wenngleich  es  einzelne  Punkte  im  Korper  gibt,  wo  der  strenge  Nachweis  noch  nicht  gelie- 
fert  ist,  doch  das  Princip  gesichert,  dass  in  der  ganzen  Reihe  alles  Lebendigen,  dies  mogen 
nun  ganze  Pflanzen  oder  thierische  Organismen  oder  integrirende  Theile  derselben  sein,  ein 
ewiges  Gesetz  der  contimdr lichen  Entwicklung  besteht."  VlRCHOW.^ 

The  law  of  genetic  cellular  continuity,  first  clearly  stated  by  Vir- 
chow  in  the  above  words,  has  now  become  one  of  the  primary  data 
of  biology.  The  cell  has  no  other  mode  of  origin  than  by  division  of 
a  pre-existing  cell.  In  the  multicellular  organism  all  the  tissue-cells 
have  arisen  by  continued  division  from  the  original  germ-cell,  and 
this  in  its  turn  arose  by  the  division  of  a  cell  pre-existing  in  the 
parent-body.  By  cell-division,  accordingly,  the  hereditary  substance 
is  split  off  from  the  parent-body  ;  and  by  cell-division,  again,  this 
substance  is  handed  on  by  the  fertilized  egg-cell  or  oosperm  to  every 
part  of  the  body  arising  from  it.^  Cell-division  is,  therefore,  one  of 
the  central  facts  of  development  and  inheritance. 

The  first  two  decades  after  Schleiden  and  Schwann  (1840-60)  were 
occupied  with  researches,  on  the  part  both  of  botanists  and  of  zool- 
ogists, which  finally  demonstrated  the  universality  of  this  process 
and  showed  the  authors  of  the  cell-theory  to  have  been  in  error  in 
asserting  the  independent  origin  of  cells  out  of  a  formative  blastema.^ 
The  mechanism  of  cell-division  was  not  precisely  investigated  until 
long  afterwards,  but  the  researches  of  Remak  ('41),  Kolliker  ('44), 
and  others  showed  that  an  essential  part  of  the  process  is  a  division 
of  both  the  nucleus  and  the  cell-body.  In  1855  {I.e.,  pp.  174,  175),  and 
again  in  1858,  Remak  gave  as  the  general  result  of  his  researches 
the  following  synopsis  or  scheme  of  cell-division.  Cell-division,  he 
asserted,  proceeds  from  the  centre  toward  the  periphery.^     It  begins 

1  Cellidarpathologie,  p.  25,  1858. 
'  Cf.  Introduction,  p.  9. 

s  For  a  full  historical  account  of  this  period,  see  Remak,  Untersuchiingen  iiber  die  Ent- 
-cvicklung  der  Wirbelthiere,  1855,  pp.  164-180. 
■*  Untersiichiingen,  p.  175. 

45 


46 


CELL-DIVISION 


with  the  division  of  the  nucleolus,  is  continued  by  simple  constriction 
and  division  of  the  nucleus,  and  is  completed  by  division  of  the  cell- 
body  and  membrane  (Fig.  i8).  For  many  years  this  account  was 
accepted,  and  no  essential  advance  beyond  Remak's  scheme  was 
made  for  nearly  twenty  years.  A  number  of  isolated  observations 
were,  however,  from  time  to  time  made,  even  at  a  very  early  period, 
which  seemed  to  show  that  cell-division  was  by  no  means  so  sim- 
ple an  operation  as  Remak  believed.  In  some  cases  the  nucleus 
seemed  to  disappear  entirely  before  cell-division  (the  germinal  vesicle 
of  the  ovum,  according  to  Reichert,  Von  Baer,  Robin,  etc.);  in  others 
to  become  lobed  or  star-shaped,  as  described  by  Virchow  and  by 
Remak  himself  (Fig.  i8,/).  It  was  not  until  1873  that  the  way  was 
opened  for  a  better  understanding  of  the  matter.     In  this  year  the 

discoveries  of  Anton  Schneider, 
quickly  followed  by  others  in 
the  same  direction  by  Biitschli, 
Fol,  Strasburger,  Van  Beneden, 
Flemming,  and  Hertwig,  showed 
cell-division  to  be  a  far  more 
elaborate  process  than  had  been 
supposed,  and  to  involve  a  com- 
plicated transformation  of  the 
nucleus  to  which  Schleicher 
i^J^)  afterwards  gave  the  name 
of  Karyokinesis.  It  soon  ap- 
peared, however,  that  this  mode 
of  division  was  not  of  univer- 
sal occurrence  ;  and  that  cell- 
division  is  of  two  widely  different  types,  which  Van  Beneden  (^76) 
distinguished  as  fraginejttatiofiy  corresponding  nearly  to  the  simple 
process  described  by  Remak,  and  division,  involving  the  more  com- 
plicated process  of  karyokinesis.  Three  years  later  Flemming  ('79) 
proposed  to  substitute  for  these  the  terms  direct  and  indirect  division, 
which  are  still  used.  Still  later  ('82)  the  same  author  suggested  the 
terms  mitosis  (indirect  or  karyokinetic  division)  and  amitosis  (direct 
or  akinetic  division),  which  have  rapidly  made  their  way  into  general 
use,  though  the  earlier  terms  are  often  employed. 

Modern  research  has  demonstrated  the  fact  that  amitosis  or  direct 
division,  regarded  by  Remak  and  his  immediate  followers  as  of  uni- 
versal occurrence,  is  in  reality  a  rare  and  exceptional  process ;  and 
there  is  reason  to  believe,  furthermore,  that  it  is  especially  char- 
acteristic of  highly  specialized  cells  incapable  of  long-continued 
multiplication  or  such  as  are  in  the  early  stages  of  degeneration, 
for    instance,  in   glandular  epithelia,  in   the  cells   of    transitory   em- 


0. 


d  -  € 

Fig.  18.  —  Direct  division  of  blood-cells  in 
the  embryo  chick,  illustrating  Remak's  scheme. 
[Remak.] 

a-e.  Successive  stages  of  division;  f.  Cell 
dividing  by  mitosis. 


OUTLINE    OF  INDIRECT  DIVISION  OR  MITOSIS  47 

bryonic  envelopes,  and  in  tumours  and  other  pathological  forma- 
tions, where  it  is  of  frequent  occurrence.  Whether  this  view  be 
well  founded  or  not,  it  is  certain  that  in  all  the  higher  and  in  many 
of  the  low€r  forms  of  life,  indirect  division  or  mitosis  is  the  typical 
mode  of  cell-division.  It  is  by  mitotic  division  that  the  germ-cells 
arise  and  are  prepared  for  their  union  during  the  process  of  matura- 
tion, and  by  mitotic  division  the  oosperm  segments  and  gives  rise 
to  the  tissue-cells.  It  occurs  not  only  in  the  highest  forms  of  plants 
and  animals,  but  also  in  such  simple  forms  as  the  Rhizopods,  Flagel- 
lates, and  Diatoms.  We  may,  therefore,  justly  regard  it  as  the  most 
general  expression  of  the  ''eternal  law  of  continuous  development" 
on  which  Virchow  insisted. 


A.     Outline  of  Indirect  Division  or  Mitosis  (Karyokinesis) 

The  process  of  mitosis  involves  three  parallel  series  of  changes 
which  affect  the  nucleus,  the  centrosome,  and  the  cytoplasm  of  the 
cell-body  respectively.  For  descriptive  purposes  it  may  conveniently 
be  divided  into  a  series  of  successive  stages  or  phases,  which,  how- 
ever, graduate  into  one  another  and  are  separated  by  no  well-defined 
limits.  These  are:  (i)  The  Pi'opJiases,  or  preparatory  changes; 
(2)  the  Metaphase,  which  involves  the  most  essential  step  in  the 
division  of  the  nucleus;  (3)  the  Anaphases,  in  which  the  nuclear 
material  is  distributed;  (4)  the  Telophases,  in  which  the  entire  cell 
divides  and  the  daughter-cells  are  formed. 

I.  Prophases.  — {a)  The  Nucleus.  As  the  cell  prepares  for  division 
the  most  conspicuous  fact  is  a  transformation  of  the  nuclear  sub- 
stance, involving  both  physical  and  chemical  changes.  The  chroma- 
tin resolves  itself  little  by  little  into  a  more  or  less  convoluted  thread, 
known  as  the  skei7i  (Knauel)  or  spireme,  and  its  substance  stains  far 
more  intensely  than  that  of  the  reticulum  (Fig.  19).  In  some 
cases  there  is  but  a  single  continuous  thread ;  in  others,  the  thread 
is  from  its  first  appearance  divided  into  a  number  of  separate  pieces 
or  segments  forming  a  segmented  spireme.  In  either  case  it  ulti- 
mately breaks  transversely  into  a  definite  number  of  distinct  bodies, 
known  as  chromosomes  (Waldeyer,  '88),  which  in  most  cases  have 
the  form  of  rods,  straight  or  curved,  though  they  are  sometimes 
spherical  or  ovoidal,  and  in  certain  cases  may  be  joined  together 
in  the  form  of  rings.  The  staining  power  of  the  chromatin  is  now 
at  a  maximum.  As  a  rule  the  nuclear  membrane  meanwhile  fades 
away  and  finally  disappears.  The  chromosomes  now  lie  naked  in  the 
cell,  and  the  ground-substance  of  the  nucleus  becomes  continuous 
with  the  surrounding  cytoplasm  (Fig.  19,  D,  E,  F). 


48 


CELL-DIVISION' 


Every  species  of  plaiit  or  animal  has  a  fixed  and  cJiaracteristic  num- 
ber of  cliromosomcs^  which  regularly  7rcnrs  in  the  division  of  all  of  its 
cells;  and  in  all  forms  arising  by  sexual  reproduction  the  number  is 


E 


Fig.  19,  —  Diagrams  showing  the  prophases  of  mitosis. 
A.  Resting-cell  with  reticular  nucleus  and  true  nucleolus ;  at  c  the  attraction-sphere  contain- 
ing two  centrosomes.  B.  Early  prophase  ;  the  chromatin  forming  a  continuous  spireme,  nucleolus 
still  present;  above,  the  amphiaster  {a).  C.  D.  Two  different  types  of  later  prophases;  C.  Dis- 
appearance of  the  primary  spindle,  divergence  of  the  centrosomes  to  opposite  poles  of  the  nucleus 
(examples,  many  plant-cells,  cleavage-stages  of  many  eggs),  D.  Persistence  of  the  primary 
spindle  (to  form  in  some  cases  the  "  central  spindle"),  fading  of  the  nuclear  membrane,  ingrowth 
of  the  astral  rays,  segmentation  of  the  spireme-thread  to  form  the  chromosomes  (examples,  epi- 
dermal cells  of  salamander,  formation  of  the  polar  bodies).  E.  Later  prophase  of  type  C\  fading 
of  the  nuclear  membrane  at  the  poles,  formation  of  a  new  spindle  inside  the  nucleus;  precocious 
splitting  of  the  chromosomes  (the  latter  not  characteristic  of  this  type  alone).  F.  The  mitotic 
figure  established;    e.p.  The  equatorial  plate  of  chromosomes.     (Cf.  Figs.  16,  21,  24.) 


OUTLINE    OF  INDIRECT  DIVISION   OR  MITOSIS  49 

even.  Thus,  in  some  of  the  sharks  the  number  is  36;  in  certain 
gasteropods  it  is  32  ;  in  the  mouse,  the  salamander,  the  trout,  the  lily, 
24 ;  in  the  worm  Sagitta,  1 8  ;  in  the  ox,  guinea-pig,  and  in  man  the 
number  is  said  to  be  16,  and  the  same  number  is  characteristic  of  the 
onion.  Irt  the  grasshopper  it  is  12  ;  in  the  hepatic  Pallavicinia  and 
some  of  the  nematodes,  8  ;  and  in  Ascaris^  another  thread-worm,  4  or 
2.  In  the  crustacean  Artemia  it  is  168.^  Under  certain  conditions, 
it  is  true,  the  number  of  chromosomes  may  be  less  than  the  normal 
in  a  given  species ;  but  these  variations  are  only  apparent  exceptions 
(p.  61).  The  even  number  of  chromosomes  is  a  most  interesting 
fact,  which,  as  will  appear  hereafter  (p.  135),  is  due  to  the  derivation 
of  one-half  the  number  from  each  of  the  parents. 

The  nucleoli  differ  in  their  behaviour  in  different  cases.  Net-knots, 
consisting  of  true  chromatin,  probably  enter  into  the  formation  of  the 
spireme-thread.  True  nucleoli  seem  to  dissolve  and  disappear,  or  in 
some  cases  are  cast  out  bodily  into  the  cytoplasm,  where  they  degen- 
erate and  have  no  further  function.  Whether  they  ever  contribute 
to  the  formation  of  chromosomes  is  uncertain. 

{h)  The  AmpJiiaster.  Meanwhile,  more  or  less  nearly  parallel  with 
these  changes  in  the  chromatin,  a  complicated  structure  known  as  the 
innpJiiaster  (Fol,  'jj)  makes  its  appearance  in  the  position  formerly 
occupied  by  the  nucleus  (Fig.  19,  B-F).  This  structure  consists 
of  a  fibrous  spindle-shaped  body,  the  spindle^  at  either  pole  of  which 
is  a  star  or  aster  formed  of  rays  or  astral  fibres  radiating  into  the  sur- 
rounding cytoplasm,  the  whole  strongly  suggesting  the  arrangement 
of  iron  filings  in  the  field  of  a  horseshoe  magnet.  The  centre  of  each 
aster  is  occupied  by  a  minute  body,  known  as  the  centrosome  (Boveri, 
"^'^),  which  may  be  surrounded  by  a  spherical  mass  known  as  the 
centrospJiere  (Strasburger,  '93).  As  the  amphiaster  forms,  the  chro- 
mosomes group  themselves  in  a  plane  passing  through  the  equator  of 
the  spindle,  and  thus  form  what  is  known  as  the  equatorial  plate. 

The  amphiaster  arises  under  the  influence  of  the  centrosome  of  the 
resting-cell,  which  divides  into  two  similar  halves,  an  aster  being 
developed  around  each  while  a  spindle  stretches  between  them  (Fig. 
19,  A-D\  In  most  cases  this  process  begins  outside  the  nucleus,  but 
the  subsequent  phenomena  vary  considerably  in  different  forms.  In 
some  forms  (tissue-cells  of  the  salamander)  the  amphiaster  at  first  lies 
tangentially  outside  the  nucleus,  and  as  the  nuclear  membrane  fades 
away,  some  of  the  astral  rays  grow  into  the  nucleus  from  the  side, 
become  attached  to  the  chromosomes,  and  finally  pull  them  into  posi- 
tion around  the  equator  of  the  spindle,  which  is  here  called  the  cen- 
tral spindle  (Figs.  19,  j9,  F\  21).     In  other  cases  the  original  spindle 

1  For  a  more  complete  list  see  p.  154- 


so 


CELL-DIVISION 


disappears,  and  the  two  asters  pass  to  opposite  poles  of  the  nucleus 
(most  plant  mitoses  and  in  many  animal  cells).  A  spindle  is  now 
formed  from  rays  that  grow  into  the  nucleus  from  each  aster,  the 
nuclear  membrane  fading  away  at  the  poles,  though  in  some  cases  it 
may  be  pushed  in  by  the  spindle-fibres  for  some  distance  before  its 


Fig.  20.  —  Diagrams  of  the  later  phases  of  mitosis. 
G.  Metaphase;  splitting  of  the  chromosomes  {e.p^\  n.  The  cast-ofF  nucleolus.  H.  Ana- 
phase; the  daughter-chromosomes  diverging,  between  them  the  interzonal  fibres  (i./.),  or  central 
spindle;  centrosomes  already  doubled  in  anticipation  of  the  ensuing  division.  /.  Late  anaphase 
or  telophase,  showing  division  of  the  cell-body,  mid-body  at  the  equator  of  the  spindle  and  begin- 
ning reconstruction  of  the  daughter-nuclei,     y.  Division  completed. 

disappearance  (Fig.  19,  C,  B).  In  this  case  there  is  apparently  no 
central  spindle.  In  a  few  exceptional  cases,  finally,  the  amphiaster 
may  arise  inside  the  nucleus  (p.  225). 

The  entire  structure,  resulting  from  the  foregoing  changes,  is 
known  as  the  karyokinetic  or  mitotic  figure.  It  may  be  described  as 
consisting  of  two  distinct  parts;  namely,  i,  the  c/womatic  figure, 
formed  by  the  deeply  staining  chromosomes ;  and,  2,  the  achromatic 


OUTLINE    OF  INDIRECr  DIVISION   OR   MITOSIS  5 1 

figitre,  consisting  of  the  spindle  and  asters  which,  in  general,  stain 
but  slightly.  The  fibrous  substance  of  the  achromatic  figure  is  gener- 
ally known  as  aixJioplasm  (Boveri,  "^'^\  but  this  term  is  not  applied 
to  the  centrosome  within  the  aster. 

2.  Metaphase.  —  T\iQ  propJiases  of  mitosis  are,  on  the  whole,  pre- 
paratory in  character.  The  metaphase,  which  follows,  forms  the 
initial  phase  of  actual  division.  Each  chromosome  splits  lengthwise 
into  two  exactly  similar  halves,  which  afterwards  diverge  to  opposite 
poles  of  the  spindle,  and  here  each  group  of  daughter-chromosomes 
finally  gives  rise  to  a  daughter-nucleus  (Fig.  20).  In  some  cases 
the  splitting  of  the  chromosomes  cannot  be  seen  until  they  have 
grouped  themselves  in  the  equatorial  plane  of  the  spindle ;  and  it  is 
only  in  this  case  that  the  term"  "metaphase"  can  be  applied  to  the 
mitotic  figure  as  a  whole.  In  a  large  number  of  cases,  however,  the 
splitting  may  take  place  at  an  earlier  period  in  the  spireme  stage,  or 
even,  in  a  few  cases,  in  the  reticulum  of  the  mother-nucleus  (Figs. 
38,  39).  Such  variations  do  not,  however,  affect  the  essential  fact 
that  tJie  cJiromatic  netivork  is  converted  into  a  thread^  zvhich,  zvhether 
continuous  or  discontinuous,  splits  tJiroughout  its  entire  length  into 
two  exactly  equivalejit  halves.  The  splitting  of  the  chromosomes, 
discovered  by  Flemming  in  1880,  is  the  most  significant  and  funda- 
mental operation  of  cell-division ;  for  by  it,  as  Roux  first  pointed  out 
('83),  the  entire  substance  of  the  chromatic  network  is  precisely  halved, 
and  the  daughter-nuclei  receive  precisely  equivalent  portiofts  of  cJiro- 
matin  fro7n  the  mother-nucleiLS.  It  is  very  important  to  observe  that 
the  nuclear  division  always  shows  this  exact  equality,  whether  division 
of  the  cell-body  be  equal  or  unequal.  The  minute  polar  body,  for 
example  (p.  131),  receives  exactly  the  same  amount  of  chromatin  as 
the  ^gg,  though  the  latter  is  of  gigantic  size  as  compared  with  the 
former.  On  the  other  hand,  the  size  of  the  asters  varies  with  that 
of  the  daughter-cells  (cf.  Figs.  43,  71)  though  not  in  strict  ratio. 
The  fact  is  one  of  great  significance  for  the  general  theory  of  mitosis, 
as  will  appear  beyond. 

3.  AnapJiases.  —  After  splitting  of  the  chromosomes,  the  daughter- 
chromosomes,  arranged  in  two  corresponding  groups,^  diverge  to  oppo- 
site poles  of  the  spindle,  where  they  become  closely  crowded  in  a  mass 
near  the  centre  of  the  aster.  As  they  diverge,  the  two  groups  of 
daughter-chromosomes  are  connected  by  a  bundle  of  achromatic 
fibres,  stretching  across  the  interval  between  them,  and  known  as  the 
interzonal  fibres  or  connecting  fibres.^     In  some  cases,  these  differ  in  a 

1  It  was  this  fact  that  led  Flemming  to  employ  the  word  "  mitosis"  (fxiros,  a  thread). 

2  This  stage  is  termed  by  Flemming  the  dyaster,  a  term  which  should,  however,  be  aban- 
doned in  order  to  avoid  confusion  with  the  earlier  word  ampJiiaster.  The  latter  convenient 
and  appropriate  term  clearly  has  priority. 

3  VerbindiDigsfaseni  of  German  authors;  filaments  rennissaiils  of  Van  Beneden. 


52  CELL-DIVISION 

marked  degree  from  the  other  spindle-fibres ;  and  they  are  believed 
by  many  observers  to  have  an  entirely  different  origin  and  function. 
A  view  now  widely  held  is  that  of  Hermann,  who  regards  these  fibres 
as  belonging  to  a  central  spindle,  surrounded  by  a  peripheral  layer 
of  mantle-fibres  to  which  the  chromosomes  are  attached,  and  only 
exposed  to  view  as  the  chromosomes  separate.^  They  are  sometimes 
thickened  in  the  equatorial  region  to  form  a  body  known  as  the  cell- 
plate  or  mid-body^  which,  in  the  case  of  plant-cells,  takes  part  in  the 
formation  of  the  membrane  by  which  the  daughter-cells  are  separated. 

4.  Telophases.  —  In  the  final  phases  of  mitosis,  the  entire  cell 
divides  in  two  in  a  plane  passing  through  the  equator  of  the  spindle, 
each  of  the  daughter-cells  receiving  a  group  of  chromosomes,  half 
of  the  spindle,  and  one  of  the  asters  with  its  centrosome.  Meanwhile, 
a  daughter-nucleus  is  reconstructed  in  each  cell  from  the  group  of 
chromosomes  it  contains.  The  nature  of  this  process  differs  greatly 
in  different  kinds  of  cells.  Sometimes,  as  in  the  epithelial  cells  of 
amphibia,  especially  studied  by  Flemming  and  Rabl,  and  in  many 
plant-cells,  the  daughter-chromosomes  become  thickened,  contorted, 
and  closely  crowded  to  form  a  daiighter-spireme y  closely  similar  to  that 
of  the  mother-nucleus  (Fig.  23);  this  becomes  surrounded  by  a  mem- 
brane, the  threads  give  forth  branches,  and  thus  produce  a  reticular 
nucleus.  A  somewhat  similar  set  of  changes  takes  place  in  the  seg- 
menting eggs  of  Ascaris  (Van  Beneden,  Boveri).  In  other  cases,  as 
in  many  segmenting  ova,  each  chromosome  gives  rise  to  a  hollow 
vesicle,  after  which  the  vesicles  fuse  together  to  produce  a  single 
nucleus  (Fig.  37).  When  first  formed,  the  daughter-nuclei  are  of 
equal  size.  If,  however,  division  of  the  cell-body  has  been  unequal, 
the  nuclei  become,  in  the  end,  correspondingly  unequal  —  a  fact 
which,  as  Conklin  and  others  have  pointed  out,  proves  that  the  size 
of  the  nucleus  is  controlled  by  that  of  the  cytoplasmic  mass  in  which 
it  lies. 

The  fate  of  the  achromatic  structures  varies  considerably,  and  has 
been  accurately  determined  in  only  a  few  cases.  As  a  rule,  the 
spindle-fibres  disappear  more  or  less  completely,  but  a  portion  of  their 
substance  sometimes  persists  in  a  modified  form.  In  dividing  plant- 
cells,  the  interzonal  fibres  become  thickened  at  the  equator  of  the 
spindle  and  form  a  transverse  plate  of  granules,  known  as  the  cell- 
plate  (Fig.  25),  which  gives  rise  to  the  membrane  by  which  the  two 
daughter-cells  are  separated.  The  remainder  of  the  spindle  disap- 
pears. A  similar  cell-plate  occurs  in  some  animal  cells;  but  it  is 
often  greatly  reduced,  and  may  form  only  a  minute  body  known  as 
the  mid-body  (Zwischenkorper),  which  lies  between  the  two  cells  after 

1  Cf.  p.  74. 


ORIGIN   OF    THE   MITOTIC  FIGURE  53 

their  division  (Fig.  23).  In  other  cases,  as  in  the  cells  of  the  testis, 
the  remains  of  the  spindle  in  each  cell  sometimes  gives  rise  to  a  more 
or  less  definite  body  known  as  Xho.  pa7'an?ic/eus  or  Nebenkern  (Fig.  62). 

The  aster  may  in  some  cases  entirely  disappear,  together  with  the 
centrosome  (as  occurs  in  the  mature  ^g%)-  In  a  large  number  of 
cases,  however,  the  centrosome  persists,  lying  either  outside  or  more 
rarely  inside  the  nucleus  and  dividing  into  two  at  a  very  early  period. 
This  division  is  clearly  a  precocious  preparation  for  the  ensuing  divi- 
sion of  the  daughter-cell,  and  it  is  a  remarkable  fact  that  it  occurs  as 
a  rule  during  the  early  anaphase,  before  the  mother-cell  itself  has 
divided.  There  are,  however,  some  undoubted  cases  (cf.  Figs.  6,  7)  in 
which  the  centrosome  remains  undivided  during  the  resting  stage 
and  only  divides  as  the  process  of  mitosis  begins. 

Like  the  centrosome,  the  aster  or  its  central  portion  may  persist  in 
a  more  or  less  modified  form  throughout  the  resting  state  of  the  cell, 
forming  a  structure  generally  known  as  the  attractiofi-spheir.  This 
body  often  shows  a  true  astral  structure  with  radiating  fibres  (Figs.  7, 
35);  but  it  is  sometimes  reduced  to  a  regular  spherical  mass  which 
may  represent  only  the  centrosphere  of  the  original  aster  (Fig.  6). 


B.     Origin  of  the  Mitotic  Figure 

The  chromatic  figure  (chromosomes)  is  derived  directly  from  the 
chromatic  network  of  the  resting-nucleus  as  described  above.  The 
derivation  of  the  achromatic  figure  (spindle  and  asters)  is  a  far  more 
difficult  question,  which  is  still  to  some  extent  involved  in  doubt.  By 
the  earlier  observers  (1873-75)  the  achromatic  figure  was  supposed 
to  disappear  entirely  at  the  close  of  cell-division,  and  most  of  them 
(Biitschli,  Strasburger,  Van  Beneden,  '75)  believed  it  to  be  reformed 
at  each  succeeding  division  out  of  the  nuclear  substance.  Later  re- 
searches (1875-85)  gave  contradictory  and  apparently  irreconcilable 
results.  Fol  ('79)  derived  the  spindle  from  the  nuclear  material, 
the  asters  from  the  cytoplasm.  Strasburger  ('80)  asserted  that  the 
entire  achromatic  figure  arose  from  the  cytoplasm.  Flemming  ('82) 
was  in  doubt,  and  regarded  the  question  of  nuclear  or  cytoplasmic 
origin  as  one  of  minor  importance,  yet  on  the  whole  inclined  to  the 
opinion  that  the  achromatic  figure  arose  inside  the  nucleus.^  In  1887 
a  new  face  was  put  on  the  whole  question  through  the  independent 
discovery  by  Van  Beneden  and  Boveri  that  the  centrosome  does  not 
disappear  at  the  close  of  mitosis,  but  remains  as  a  distinct  cell-organ 
lying  beside  the  nucleus  in  the  cytoplasm.  These  investigators  agreed 
that  the  amphiaster  is  formed  under  the  influence  of  the  centrosome, 

1  Zclhuhsfanz,  p.  226. 


54 


CELL-DIVISION 


which  leads  the  way  in  cell-division  by  dividing  into  two  similar 
halves  to  form  the  centres  of  division.  ''Thus  we  are  justified,"  said 
Van   Beneden,   "in   rei^ardinij:   the  attraction-sphere  with  its  central 


K 


*^) 


.--X, 


Fig.  21.  —  The  prophases  in  cells   (spermatogonia  and  spermatocytes)   of  the  salamander. 

[DRUNER.] 

A.  Spermatogonium  in  the  spireme-stage ;  the  chromatin-thread  lies  in  the  linin-network,  still 
surrounded  by  the  membrane;  above,  the  two  centrosomes,  the  central  spindle  not  yet  formed. 
B.  Later  stage  (spermatocyte)  ;  the  nuclear  membrane  has  disappeared,  leaving  the  naked  chro- 
mosomes; above,  the  amphiaster,  with  centrosomes  and  central  spindle;  astral  rays  extending 
towards  the  chromosomes.  D.  Following  stage ;  splitting  of  the  chromosomes,  growth  of  the 
aster;  mantle-fibres  and  central  spindle  clearly  distinguished.  C.  The  fully  formed  mitotic  figure 
(metaphase)  ;  the  chromosomes,  fully  divided,  grouped  in  the  equatorial  plate. 

corpuscle  as  forming  a  permanent  organ,  not  only  of  the  early  blas- 
tomeres,  but  of  all  cells ;  that  it  constitutes  a  cell-organ  equal  in  rank 
to  the  nucleus  itself  ;  that  every  central  corpuscle  is  derived  from 
a  pre-existing  corpuscle,  every  attraction-sphere  from  the  pre-existing 


ORIGIN  OF   THE  MITOTIC  FIGURE 


55 


sphere,  and  that  division  of  the  sphere  precedes  that  of  the  cell- 
nucleus."^  Boveri  expressed  himself  in  similar  terms  in  the  same 
year  i^'^J,  2,  p.  153),  and  the  same  general  result  was  reached  by 
Vej do vsky  nearly  at  the  same  time,^  though  it  was  less  clearly  formu- 
lated than  by  either  Boveri  or  Van  Beneden. 


Fig.  22.  —  Metaphase  and  anaphases  of  mitosis  in  cells  (spermatocytes)  of  the  salamander. 
[Druner.] 

E.  Metaphase.  The  continuous  central  spindle-fibres  pass  from  pole  to  pole  of  the  spindle. 
Outside  them  the  thin  layer  of  contractile  mantle-fibres  attached  to  the  divided  chromosomes,  of 
which  only  two  are  shown.  Centrosomes  and  asters.  F.  Transverse  section  through  the  mitotic 
figure  showing  the  ring  of  chromosomes  surrounding  the  central  spindle,  the  cut  fibres  of  the  latter 
appearing  as  dots.  G.  Anapliase ;  divergence  of  the  daughter-chromosomes,  exposing  the  cen- 
tral spindle  as  the  interzonal  fibres;  contractile  fibres  (principal  cones  of  Van  Beneden)  clearly 
shown.  H.  Later  anaphase  (dyaster  of  Flemming)  ;  the  central  spindle  fully  exposed  to  view; 
mantle-fibres  attached  to  the  chromosomes.    Immediately  afterwards  the  cell  divides  (see  Fig.  23). 

All  these  observers  agreed,  therefore,  that  the  achromatic  figure 
arose  outside  the  nucleus,  in  the  cytoplasm  ;  that  the  primary  impulse 
to  cell-division  was  given,  not  by  the  nucleus,  but  by  the  centrosome, 
and  that  a  new  cell-organ  had  been  discovered  whose  special  office 


'87,  p.  279. 


'88,  pp.  151,  etc. 


56 


CELL-DIVISION 


was  to  preside  over  cell-division.  "  The  centrosome  is  an  indepen- 
dent permanent  cell-organ,  which,  exactly  like  the  chromatic  elements, 
is  transmitted  by  division  to  the  daughter-rr/A\  Tlie  ce7itrosome  rep- 
resents the  dynamic  centre  of  the  cell.''  ^  This  view  has  been  widely 
accepted  by  later  investigators,  and  the  centrosome  has  been  shown 
to  occur  in  a  large  number  of  adult  tissue-cells  during  their  resting 
state ;  for  example  in  pigment-cells,  leucocytes,  connective  tissue- 
cells,  epithelial  and  endothelial  cells,  in  certain  gland-cells  and  nerve- 
cells,  in  the  cells  of  many  plant-tissues,  and  in  some  of  the  unicellular 


Fig.  23.  —  Final  phases  (telophases)  of  mitosis  in  salamander  cells.  [Flemming.] 
/.  Epithelial  cell  from  the  lung;  chromosomes  at  the  poles  of  the  spindle,  the  cell-body  divid- 
ing: granules  of  the  "mid-body"  or  Z^vischenkorper  at  the  equator  of  the  disappearing  spindle. 
y.  Connective-tissue  cell  (lung)  immediately  after  division ;  daughter-nuclei  reforming,  the  cen- 
trosome just  outside  of  each ;  mid-body  a  single  granule  in  the  middle  of  the  remains  of  the 
spindle. 

plants,  and  animals,  such  as  the  Diatoms  and  Flagellates.  That 
the  centrosome  gives  the  primary  impulse  to  cell-division  by  its  own 
division  has,  however,  been  disproved ;  for  there  are  several  accu- 
rately determined  cases  in  which  the  chromatin-elements  divide 
long  before  the  centrosome,  and  it  is  now  generally  agreed  that  the 
division  of  chromatin  and  centrosome  are  two  parallel  events,  the 
causal  relation  between  which  still  remains  undetermined.  (Cf. 
P-  77) 

1  Boveri,  '87,  2,  p.  153. 


I 


MODIFICATIONS   OF  MITOSIS  57 


C.     Modifications   of    Mitosis 

The  evidence  steadily  accumulates  that  the  'essential  phenomena 
of  mitosis  are  of  the  same  general  type  in  all  forms  ot  cells,  both 
in  plants  and  in  animals.  Everywhere,  with  a  single  important 
exception  (maturation),  the  chromatin-thread  splits  lengthwise  through- 
out its  whole  extent,  and  everywhere  an  achromatic  spindle  is  formed 
that  is  in  some  manner  an  agent  in  the  transportal  of  the  chromatin- 
halves  to  the  respective  daughter-cells.  The  exception  to  this  general 
law,  which  occurs  during  the  preparation  of  the  germ-cells  for  their 
development  and  constitutes  one  of  the  most  significant  of  all  cyto- 
logical  phenomena,  is  considered  in  Chapter  V.  We  have  here  only 
to  glance  at  a  number  of  modifications  that  affect,  not  the  essential 
character,  but  only  the  details  of  the  typical  process. 

I .    Varieties  of  the  Mitotic  Figure 

All  of  the  mitotic  phenomena,  and  especially  those  involved  in  the 
history  of  the  achromatic  figure,  are  in  general  most  clearly  displayed 
in  embryonic  cells,  and  especially  in  the  egg-cell  ^  (Fig.  24).  In 
the  adult  tissue-cells  the  asters  are  relatively  small,  the  spindle 
relatively  large  and  conspicuous.  The  same  is  true  of  plant-cells 
in  general  where  the  very  existence  of  the  asters  was  at  first 
overlooked.  Plant-mitoses  are  characterized  by  the  prominence  of 
the  cell-plate  (Fig.  25),  which  is  rudimentary  or  often  wanting  in 
animals,  a  fact  correlated  no  doubt  with  the  greater  development 
of  the  cell-membrane  in  plants.  With  this  again  is  correlated  the 
fact  that  division  of  the  cell-body  in  animal-cells  generally  takes  place 
by  constriction  in  the  equatorial  plane  of  the  spindle ;  while  in  plant- 
cells  the  cell  is  usually  cut  in  two  by  a  cell-wall  developed  in  the 
substance  of  the  protoplasm  and  derived  in  large  part  from  the  cell- 
plate. 

The  centrosome  and  centrosphere  appear  to  present  great  varia- 
tions that  have  not  yet  been  thoroughly  cleared  up  and  will  be  more 
critically  discussed  beyond.^  They  are  known  to  undergo  extensive 
changes  in  the  cycle  of  cell-division  and  to  vary  greatly  in  different 
forms  (Fig.  108).  In  some  cases  the  aster  contains  at  its  centre 
nothing  more  than  a  minute  deeply  staining  granule,  which  doubtless 

^  A  very  remarkable  modification  of  the  achromatic  figure  occurs  in  the  spiral  asters^ 
discovered  by  Mark  ('81)  in  the  eggs  of  Umax,  the  astral  rays  being  curved  as  if  the  entire 
aster  had  been  rotated  about  its  centre.     The  meaning  of  this  phenomenon  is  unknown. 

^  See  p.  224. 


58 


CELL-DIVISION 


represents  the  centrosome  alone.  In  other  cases  the  granule  is  sur- 
rounded by  a  larger  body,  which  in  turn  lies  within  the  centrosphere 
or  attraction-sphere.  In  still  other  cases  the  centre  of  the  aster  is 
occupied  by  a  large  reticular  mass,  within  which  no  smaller  body  can 
be  distinguished  {e.g.  in  pigment-cells) ;  this  mass  is  sometimes  called 
the  centrosome,  sometimes  the  centrosphere.     Sometimes,  again,  the 


Fig.  24.  — The  middle  phases  of  mitosis  in  the  first  cleavage  of  the  Ascaris-egg.     [BOVERI.] 
A.  Closing  prophase,  the  equatorial  plate  forming.     B.   Metaphase ;    equatorial  plate  estab- 
lished and  the  chromosomes  split ;  d,  the  equatorial  plate,  viewed  en  face,  showing  the  four  chro- 
mosomes.    C.   Early  anaphase;    divergence  of  the  daughter-chromosomes  (polar  body  at  one 
side).     D.  Later  anaphase ;  ^.^.,  second  polar  body. 

(For  preceding  stages  see  Fig.  65 ;  for  later  stages,  Fig.  104.) 


spindle-fibres  are  not  focussed  at  a  single  point,  and  the  spindle 
appears  truncated  at  the  ends,  its  fibres  terminating  in  a  transverse 
row  of  granules  (maturation-spindles  of  Ascaris\  and  some  plant-cells). 
It  is  not  entirely  certain,  however,  that  such  spindles  observed  in 
preparations  represent  the  normal  structure  during  lifc.^ 

^  Hacker  asserts  in  a  recent  paper  ('94)  that  tlie  truncated  polar  spindles  are  normal, 
and  that  a  centrosome  lies  at  each  of  the  four  angles;  i.e.  two  at  either  pole. 


MODIFICATIONS   OF  MITOSIS 


59 


The  variations  of  the  chromatic  figure  must  for  the  most  part  be 
considered  in  the  more  special  parts  of  this  work.  There  seems 
to  be  doubt  that  a  single  continuous  spireme-thread  may  be  formed 
(cf.  p.  184)-,  but  it  is  equally  certain  that  the  thread  may  appear  from 
the  beginning  in  a  number  of  distinct  segments ;  i.e.  as  a  segmented 
spireme.  The  chromosomes,  when  fully  formed,  vary  greatly  in 
appearance.     In  many  of  the  tissues  of  adult  plants    and   animals 


Fig.  25.  -^  Division  of  pollen-mother-cells  in  the  lily.  [GUIGNARD.] 
A.  Anaphase  of  the  first  division,  showing  the  twelve  daughter-chromosomes  on  each  side,  the 
interzonal  fibres  stretching  between  them,  and  the  centrosomes,  already  double,  at  the  spindle- 
poles.  B.  Later  stage,  showing  the  cell-plate  at  the  equator  of  the  spindle  and  the  daughter- 
spiremes  (dispireme  stage  of  Flemming).  C.  Division  completed;  double  centrosomes  in  the 
resting  cell.  D.  Ensuing  division  in  progress ;  the  upper  cell  at  the  close  of  the  prophases,  the 
chromosomes  and  centrosomes  still  undivided ;  lower  cell  in  the  late  anaphase,  cell-plate  not  yet 
formed. 


they  are  rod-shaped  and  are  often  bent  in  the  middle  like  a  V  (Figs. 
21,  33).  They  often  have  this  form,  too,  in  embryonic  cells,  as  in 
the  segmentation-stages  of  the  ^g^  in  Ascaris  (Fig.  24)  and  other 
forms.  The  rods  may,  however,  be  short  and  straight  (segmenting 
eggs  of  echinoderms,  etc.),  and  may  be  reduced  to  spheres,  as  in 
the  maturation  stages  of  the  germ-cells. 


6o 


CELL-DIVISION 


2.    Heterotypical  Mitosis 

Under  this  name  Flemming  i^^f)  first  described  a  peculiar  modi- 
fication of  the  division  of  the  chromosomes  that  has  since  been  shown 
to  be  of  very  great  importance  in  the  early  history  of  the  germ-cells, 


Fig.  26.  —  Heterotypical  mitosis  in  spermatocytes  of  the  salamander.  [FLEMMING.] 
A.  Prophase,  chromosomes  in  the  form  of  scattered  rings,  each  of  which  represents  two 
daughter-chromosomes  joined  end  to  end.  B.  The  rings  ranged  about  the  equator  of  the  spindle 
and  dividing;  the  swellings  indicate  the  ends  of  the  chromosomes,  C.  The  same  viewed  from  the 
spindle-pole.  D,  Diagram  (Hermann)  showing  the  central  spindle,  asters  and  centrosomes,  and 
the  contractile  mantle-fibres  attached  to  the  rings  (one  of  the  latter  dividing). 


though  it  is  not  confined  to  them.  In  this  form  the  chromosomes 
split  at  an  early  period,  but  the  halves  remain  united  by  their  ends. 
Each  double  chromosome  then  opens  out  to  form  a  closed  ring 
(Fig.  26),  which  by  its  mode  of  origin  is  shown  to  represent  two 
daughter-chromosomes,    each    forming   half   of    the   ring,    united   by 


MODIFICATIONS    OF  MITOSIS  6 1 

their  ends.  The  ring  finally  breaks  in  two  to  form  two  U-shaped 
chromosomes  which  diverge  to  opposite  poles  of  the  spindle  as 
usual.  As  will  be  shown  in  Chapter  V.,  the  divisions  by  which 
the  germ;;pells  are  matured  are  in  many  cases  of  this  type ;  but 
the  primary  rings  here  represent  not  two  but  four  chromosomes, 
into  which  they  afterwards  break  up. 

3.    Bivalent  ajid  Plurivalent  Cliroviosovtes 

The  last  paragraph  leads  to  the  consideration  of  certain  varia- 
tions in  the  number  of  the  chromosomes.  Boveri  discovered  that  the 
species  Ascaris  megalocepJiala  comprises  two  varieties  which  differ  in 
no  visible  respect  save  in  the  number  of  chromosomes,  the  germ-nuclei 
of  one  form  (''variety  bivalens"  of  Hertwig)  having  two  chromo- 
somes, while  in  the  other  form  ("variety  univalens")  there  is  but  one. 
Brauer  discovered  a  similar  fact  in  the  phyllopod  Artemia,  the 
number  of  somatic  chromosomes  being  168  in  some  individuals,  in 
others  only  84  (p.  205). 

It  will  appear  hereafter  that  in  some  cases  the  primordial  germ- 
cells  show  only  half  the  usual  number  of  chromosomes,  and  in 
Cyclops^  the  same  is  true,  according  to  Hacker,  of  all  the  cells  of 
the  early  cleavage-stages. 

In  all  cases  where  the  number  of  chromosomes  is  apparently 
reduced  (''pseudo-reduction  "  of  Riickert)  it  is  highly  probable  that 
each  chromatin-rod  represents  not  one  but  two  or  more  chromosomes 
united  together,  and  Hacker  has  accordingly  proposed  the  terms 
"bivalent"  and  "plurivalent"  for  such  chromatin-rods.^  The 
truth  of  this  view,  which  originated  with  vom  Rath,  is,  I  think, 
conclusively  shown  by  the  case  of  Artemia  described  at  p.  203,  and 
by  many  facts  in  the  maturation  of  the  germ-cells  hereafter  con- 
sidered. In  Ascaris  we  may  regard  the  chromosomes  of  Hertwig's 
"variety  univalens"  as  really  bivalent  or  double;  i.e.  equivalent 
to  two  such  chromosomes  as  appear  in  "variety  bivalens."  These 
latter,  however,  are  probably  in  their  turn  plurivalent,  i.e.  represent  a 
number  of  units  of  a  lower  order  united  together;  for,  as  described  at 
p.  I II,  each  of  these  normally  breaks  up  in  the  somatic  cells  into  a 
large  number  of  shorter  chromosomes  closely  similar  to  those  of  the 
related  species  Ascaris  bimbricoidcs,  where  the  normal  number  is  24. 

^  The  words  "  bivalent "  and  "  univalent "  have  been  used  in  precisely  the  opposite  sense 
by  Hertwig  in  the  case  of  Ascaris^  the  former  term  being  applied  to  that  variety  having  two 
chromosomes  in  the  germ-cells,  the  latter  to  the  variety  with  one.  These  terms  certainly  have 
priority,  but  were  applied  only  to  a  specific  case.  Hacker's  use  of  the  words,  which  is 
strictly  in  accordance  with  their  etymology,  is  too  valuable  for  general  descriptive  purposes 
to  be  rejected. 


62 


CELL-DIVISION 


Hacker  has  called  attention  to  the  striking  fact  that  plurivalent 
mitosis  is  very  often  of  the  heterotypical  form,  as  is  very  common  in 
the  maturation  mitoses  of  many  animals  (Chapter  V.),  and  often 
occurs  in  the  early  cleavages  of  Ascaris ;  but  it  is  doubtful  whether 
this  is  a  universal  rule. 

4.    Mitosis  in  the  Unicellular  Plants  and  Animals 

The  process  of  mitosis  in  the  one-celled  plants  and  animals  has  a 
peculiar  interest,  for  it  is  here  that  we  must  look  for  indications  of 


B 


Fig.  27.  —  Mitotic  division  in  Infusoria.     [R.  Hertwig.] 
A-C.  Macronucleus  of  Spirochona,   showing   pole-plates.     D-H.   Successive  stages  in   the 
division  of  the  micronucleus  of /'araw<^<;i«;«.    D.  The  earliest  stage,  showing  reticulum.     G.  Fol- 
lowing stage  ("  sickle-form  ")  with  nucleolus.    E.  Chromosomes  and  pole-plates.    F.  Late  ana- 
phase,   H.  Final  phase. 

its  historical  origin.  But  although  traces  of  mitotic  division  were 
seen  in  the  Infusoria  by  Balbiani  ('58-'6i),  Stein  ('59),  and  others 
long  before  it  was  known  in  the  higher  forms,  it  is  still  imperfectly 
understood  on  account  of  the  practical  difficulties  of  observation. 
Within  a  few  years,  however,  our  knowledge  in  this  field  has  rapidly 
advanced,  and  we  have  already  good  ground  for  some  important 
conclusions. 

Mitotic  division  has  now  been  observed  in  many  of  the  main  divi- 
sions of  Protozoa  and  unicellular  plants ;  but  in  the  present  state  of 
the  subject  it  must  be  left  an  open  question  whether  it  occurs  in  all. 


MODIFICATIONS   OF  MITOSIS 


63 


The  essential  features  of  the  process  appear  to  be  here  of  the  same 
nature  as  in  the  higher  types,  but  show  a  series  of  minor  modifications 
that  indicate  the  origin  of  mitotic  division  from  a  simpler  type. 
Four  of  these  modifications  are  of  especial  importance,  viz. :  — 

(i)  The^  centrosome  or  its  equivalent  lies  as  a  rule  inside  the 
nucleus,  thus  reversing  the  rule  in  higher  forms. 

(2)  The  nuclear  membrane  as  a  rule  remains  intact  and  does  not 
disappear  at  any  stage. 


,f(^f>^ 


Fig.  28.  —  Mitosis  in  the  rhizopod,  Euglypha.     [SCHEWIAKOFF.] 
In  this  form  the  body  is  surrounded  by  a  firm  shell  which  prevents  direct  constriction  of  the 
cell-body.     The  latter  therefore  divides  by  a  process  of  budding  from  the  opening  of  the  shell 
(the  initial  phase  shown  at  A)  ;  the  nucleus  meanwhile  divides,  and  one  of  the  daughter-nuclei 
afterwards  wanders  out  into  the  bud, 

A.  Early  prophase ;  nucleus  near  lower  end  containing  a  nucleolus  and  numerous  chromo- 
somes. B.  Equatorial  plate  and  spindle  formed  inside  the  nucleus;  pole-bodies  or  pole-plates 
{i.e.  attraction-spheres  or  centrosomes)  at  the  spindle-poles.  C.  Metaphase.  D.  Late  ana- 
phase, spindle  dividing;  after  division  of  the  spindle  the  outer  nucleus  wanders  out  into  the  bud. 


(3)  The  asters  attain  but  a  slight  development,  and  in  some  cases 
appear  to  be  entirely  absent  (Infusoria). 

(4)  The  arrangement  of  the  chromatin-granules  to  form  chromo- 
somes appears  to  be  of  secondary  importance  as  compared  with 
higher  forms,  and  the  essential  feature  in  nuclear  division  appears  to 
be  the  fission  of  the  individual  granules. 

-  The  basis  of  our  knowledge  in  this  field  was  laid  by  Richard 
Hertwig  through  his  studies  on  an  infusorian,  SpirocJiona  ('77),  and  a 
rhizopod,  ActinospJicerium  ('84).     In  both  these  forms  a  typical  spin- 


64 


CELL-DIVISION 


die  and  equatorial  plate  are  formed  inside  the  nuclear  membrane  b}- 
a  transformation  of  the  nuclear  substance.  In  SpiroeJiona  (Fig.  27, 
A-C)  a  hemispherical  "  end-plate "  or  "  pole-plate "  is  situated  at 
either  pole  of  the  spindle,  and  Hertwig's  observations  indicated, 
though  they  did  not  prove,  that  these  plates  arose  by  the  division  of  a 
large  "nucleolus."  Pole-plates  of  a  somewhat  different  form  were 
also  described  in  Actinosphceritmi,  and  somewhat  later  by  Schewiakoff 
{^^^^  in  Eiiglypha  (Fig.  28).  Their  origin  through  division  of  the 
"  nucleolus "    has  since  been  demonstrated   by  Keuten   in  Eiiglena 


Fig.  29.  —  Mitosis  in  the  Flagellate  Euglena.     [KEUTEN.] 
A.  Preparing  for  division ;  the  nucleus  contains  a  "  nucleolus  "  or  nucleolo-centrosome  sur- 
rounded by  a  group  of  chromosomes,     B.  Division  of  the  "  nucleolus  "  to  form  an  intra-nuclear 
spindle.    C.  Later  stage.     D.  The  nuclear  division  completed. 


(Fig.  29)  and  Schaudinn  in  Amoeba.  There  can  therefore  be  little 
doubt  that  the  ''  nucleolus "  in  these  forms  represents  an  intra- 
nuclear centrosome,  and  that  the  pole-plates  are  the  daughter-centro- 
somes  or  attraction-spheres.  Richard  Hertwig's  latest  work  ('95) 
indicates  that  a  similar  process  occurs  in  the  micronuclei  of  Para- 
7ncecitmi,  which  at  first  contain  a  large  '*  nucleolus  "  and  afterwards 
a  conspicuous  pole-plate  at  either  end  of  the  spindle  (Fig.  27,  D-H). 
The  origin  of  the  pole-plates  was  not,  however,  positively  determined. 
These  facts  indicate,  as  Richard  and  Oscar  Hertwig  have  con- 
cluded, that  the  centrosome,  in  its  most  primitive  form,  is  an  intra- 


MODIFICATIONS   OF  MITOSIS 


65 


nuclear  structure,  which  may  have  arisen  through  a  condensation 
or  differentiation  of  the  "achromatic"  constituents.  Noctihica,  the 
diatoms,  and  ActinospJicerum  seem  to  represent  transitions  to  the 
higher  tyo.es.  In  the  latter  form  Brauer  discovered  a  distinct  cen- 
trosome  lying  in  the  late  anaphase  outside  the  nuclear  membrane  at 
the  centre  of  a  small  but  distinct  aster  and  soon  dividing  into  two, 
precisely  as  in  higher  forms  (Fig.  31,  /,  y\  This  centrosome,  how- 
ever, as  Brauer  infers,  lies  within 
the  nucleus  during  the  resting  state 
and  the  earlier  stages  of  division, 

and    only   migrates    out    into    the  s 

cytoplasm  during  the  late  ana- 
phase, afterward  returning  to  the 
nucleus  and  lying  in  the  "  pole- 
plate."  In  the  diatoms  Biitschli 
discovered  an  extra-nuclear  centro- 
some and  attraction-sphere,  and 
Lauterborn  has  traced  the  forma- 
tion of  a  central  spindle  from  it. 
This  spindle,  at  first  extra-nuclear, 
is  asserted  to  pass  subsequently 
into  the  interior  of  the  nucleus. 

Noctiluca^  finally,  appears  to 
have  attained  the  condition  char- 
acteristic of  the  higher  forms. 
Here,  as  Ishikawa  has  shown,  the 
cell  contains  a  typical  extra-nuclear 
centrosome  and  attraction-sphere 
lying  in  the  cytoplasm,  precisely 
as  in  Ascaris  (Fig.  30).  By  divi- 
sion of  centrosome  and  sphere  a 
typical  central  spindle  is  formed, 
about  which  the  nucleus  wraps  it- 
self, and  mitosis  proceeds  much  as 
in  the  higher  types,  except  that 
the  nuclear  membrane  does  not  disappear.  ^ 

Regarding  the  history  of  the  chromatin  the  most  thorough  obser- 
vations have  been  made  by  Schewiakoff  in  EitglypJia  and  Brauer  in 
ActinospJicBvinni.  In  the  former  case  a  segmented  spireme  arises  from 
the  resting  reticulum,  and  long,  rod-shaped  chromosomes  are  formed, 
which  are  stated  to  split  lengthwise  as  in  the  usual  forms  of  mitosis. 
The  nuclear   membrane  persists  throughout,  and  the  entire  mitotic 

1  AH  of  the  essential  features  in  this  process,  as  described  by  Ishikawa,  have  been  con- 
Hrmed  by  Calkins  in  the  Columbia  laboratory. 


Fig.  30.  —  Mitosis  in  the  Flagellate  A^<?^/i- 
luca. 

A.  Nucleus  («)  in  the  early  prophase; 
outside  it  the  attraction-sphere  (j),  containing 
two  centrosomes  (Ishikawa).  B.  The  mitotic 
figure  ;  n.  the  nucleus,  containing  rod-shaped 
chromosomes;  s.  attraction-sphere;  s.p.  ex- 
tra-nuclear central  spindle.  (Drawn  by  G.  N. 
Calkins  from  one  of  his  own  preparations.) 


es 


CELL-DIVISION 


figure,  except  the  minute  asters,  is  formed  inside  it  (Fig.  28).  In 
ActinospJicBrium,  on  the  other  hand,  there  is  no  true  spireme  stage,  and 
no  rod-shaped  chromosomes  are  at  first  formed.  The  reticulum  breaks 
up  into  a  large  number  of  granules  which  give  rise  to  an  equatorial 
plate,  divide  by  fission,  and  are  distributed  to  the  daughter-nuclei. 


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Fig.  31.  —  Mitosis  in  the  rhizopod  ActinosfhcBriutn.     [Brauer.] 

A.  Nucleus  and  surrounding  structures  in  the  early  prophase ;  above  and  beiow  the  reticular 
nucleus  lie  the  semilunar  "  pole-plates,"  and  outside  these  the  cytoplasmic  masses  in  which  the 
asters  afterward  develop.  D.  Later  stage  of  the  nucleus.  D.  Mitotic  figure  in  the  metapliase, 
showing  equatorial  plate,  intra-nuclear  spindle,  and  pole-plates  {p.p.).  C  Equatorial  plate, 
viewed  en  f-ice,  consisting  of  double  chromatin-granuU-s.  E.  Early  anaphase,  F.  G.  Later  ana- 
phases. //.  Final  anaphase.  /.  Telophase ;  daughter-nucleus  forming,  chromatin  in  loop-shaped 
threads;  outside  the  nuclear  membrane  the  centrosome,  already  divided,  and  the  aster.  J.  Later 
stage;  the  daughter-nucleus  established ;  divergence  of  the  centrosomes.  Beyond  this  point  the 
centrosomes  have  not  been  followed. 


Only  in  the  late  anaphase  {telophase^  do  these  grannies  arrange  them- 
selves in  threads  (Fig.  31,/),  and  this  process  is  apparently  no  more  than 
a  foreriinner  of  the  reticular  stage.  This  case  is  a  very  convincing 
argument  in  favour  of  the  view  that  the  formation  and  splitting  of  chro- 
mosomes is  secondary  to  the  divisi( 


n  of  the  ultimate  chromatin-granules. 


MODIFICATIONS   OF  MITOSIS 


67 


(Cf.  pp.  y'^  and  221.)  Richard  Hertwig's  studies  on  Infusoria  and 
those  of  Lauterborn  on  Flagellates  indicate  that  here  also  no  longitu- 
dinal splitting  of  the  chromatin-threads  occurs  and  that  the  division 
must  be  referred  to  the  individual  chromatin-granules.  Ishikawa  de- 
scribes a  peculiar  longitudinal  splitting  of  chromosomes  in  Noctilnca, 
but  Calkins'  studies  indicate  that  the  latter  observer  has  probably  mis- 
interpreted certain  stages  and  that  the  division  probably  takes  place  in 
a  somewhat  different  manner.  A  typical  spireme  and  chromosome- 
formation  has  also  been  described  by  Lauterborn  in  the  Diatoms  ('93). 

In  none  of  the  foregoing  cases  does  the  nuclear  membrane  dis- 
appear. In  the  gregarines,  however,  the  observations  of  Wolters 
('91)  and  Clarke  ('95)  indicate  that  the  membrane  does  not  persist, 
and  that  a  perfectly  typical  mitotic  figure  is  formed. 

To  sum  up :  The  facts  at  present  known  indicate  that  the  unicellu- 
lar forms  exhibit  forms  of  mitosis  that  are  in  some  respects  transi- 
tional from  the  typical  mitosis  of  higher  forms  to  a  simpler  type. 
The  asters  may  be  reduced  (Rhizopods)  or  wanting  (Infusoria) ;  the 
spindle  is  typically  formed  inside  the  nucleus,  either  by  division  of  an 
intra-nuclear  "  nucleolo-centrosome  "  (^Euglena^  Ainceba),  or  possibly 
by  rearrangement  of  the  chromatic  substance  without  a  differentiated 
centrosome  (.''micronuclei  of  Infusoria).  In  every  case  the  essential 
fact  in  the  history  of  the  chromatin  is  a  division  of  the  chromatin- 
granules  ;  but  this  may  be  preceded  by  their  arrangement  in  threads 
or  chromosomes  {E7tglyp/ia,  Diatoms)  or  may  not  (^Actinosphceriimi). 
TJiese  facts  point  towards  the  concltisioji  that  centrosome,  spindle,  and 
chromosomes  are  all  secondary  differejitiations  of  the  primitive  nuclear 
structure,  and  indicate  that  the  asters  and  attraction-spheres  may  be 
historically  a  later  acquisition  developed  in  the  cytoplasm  after  the  dif- 
ferentiation of  the  centrosome. 


5.    Pathological  Mitoses 

Under  certain  circumstances  the  delicate  mechanism  of  cell-division 
may  become  deranged,  and  so  give  rise  to  various  forms  of  patholog- 
ical mitoses.  Such  a  miscarriage  may  be  artificially  produced,  as 
Hertwig,  Galeotti,  and  others  have  shown,  by  treating  the  dividing- 
cells  with  poisons  and  other  chemical  substances  (quinine,  chloral, 
nicotine,  potassic  iodide,  etc.).  Pathological  mitoses  may,  however, 
occur  without  discoverable  external  cause ;  and  it  is  a  very  interest- 
ing fact,  as  Klebs,  Hansemann,  and  Galeotti  have  especially  pointed 
out,  that  they  are  of  frequent  occurrence  in  abnormal  growths  such 
as  cancers  and  tumours. 

The  abnormal  forms  of  mitoses  are  arranged  by  Hansemann  in  two 


68 


CELL-DIVISION 


general  groups,  as  follows:  (i)  asymmetrical  mitoses,  in  which  the 
chromosomes  are  unequally  distributed  to  the  daughter-cells,  and  (2) 
multipolar  mitoses,  in  which  the  number  of  centrosomes  is  more  than 
two,  and  more  than  one  spindle  is  formed.  Under  the  first  group 
are  included  not  only  the  cases  of  unequal  distribution  of  the  daugh- 
ter-chromosomes, but  also  those  in  which  chromosomes  fail  to  be 
drawn  into  the  equatorial  plate  and  hence  are  lost  in  the  cytoplasm. 

Klebs  first  pointed  out  the  occurrence  of  asymmetrical  mitoses  in 
carcinoma  cells,  where  Hiey  have  been  carefully  studied  by  Hanse- 


Fig.  32.  —  Pathological  mitoses  in  human  cancer-cells.     [Galeotti.] 
A.  Asymmetrical  mitosis  with  unequal  centrosomes.     B.  Later  stage,  showing  unequal  dis- 
tribution of  the  chromosomes.     C.  Quadripolar  mitosis.    D.  Tripolar  mitosis.    E.  Later  stage. 
F.  Tri-nucleate  cell  resulting. 


mann  and  Galeotti.  The  inequality  is  here  often  extremely  marked, 
so  that  one  of  the  daughter-cells  may  receive  more  than  twice  as 
much  chromatin  as  the  other  (Fig.  32).  Hansemann,  whose  conclu- 
sions are  accepted  by  Galeotti,  believes  that  this  asymmetry  of  mito- 
sis gives  an  explanation  of  the  familiar  fact  that  in  cancer-cells  many 
of  the  nuclei  are  especially  rich  in  chromatin  (hyper-chromatic  cells), 
while  others  are  abnormally  poor  (hypochromatic  cells).  Lustig  and 
Galeotti  ('93)  showed  that  the  unequal  distribution  of  chromatin  is 
correlated  with  and  probably  caused  by  a  corresponding  inequality 
in  the  centrosomes  which  causes  an  asymmetrical  development  of  the 
amphiaster.     A  very  interesting  discovery  made  by  Galeotti  ('93)  is 


MODIFICATIONS   OF  MITOSIS 


69 


that  asymmetrical  mitoses,  exactly  like  those  seen  in  carcinoma,  may 
be  artificially  produced  in  the  epithelial  cells  of  salamanders  (Fig. 
33)  by  treatment  with  dilute  solutions  of  various  drugs  (antipyrin, 
cocaine,  (quinine). 

Normal  multipolar  mitoses,  though  rare,  sometimes  occur,  as  in  the 
division  of  the  pollen  mother-cells  and  the  endosperm-cells  of  flower- 
ing plants  (Strasburger);  but  such  mitotic  figures  arise  through  the 
union  of  two  or  more  bipolar  amphiasters  in  a  syncytium  and  are 
due  to  a  rapid  succession  of  the  nuclear  divisions  unaccompanied  by 
fission  of  the  cell-substance.  These  are  not  to  be  confounded  with 
pathological  mitoses  arising  by  premature  or  abnormal  division  of  the 
centrosome.  If  one  centrosome  divide,  while  the  other  does  not, 
triasters    are   produced,   from  which  may  arise  three  cells  or  a  tri- 


Fig.  33.  —  Pathological  mitoses  in  epidermal  cells  of  salamander  caused  by  poisons. 
[Galeotti.] 

A.  Asymmetrical  mitosis  after  treatment  with  0.05%  antipyrin  solution.  B.  Tripolar  mitosis 
after  treatment  with  0.5%  potassic  iodide  solution. 


nucleated  cell.  If  both  centrosomes  divide  tetrasters  or  polyasters 
are  formed.  Here  again  the  same  result  has  been  artificially  attained 
by  chemical  stimulus  (cf.  Schottlander,  '88).  Multipolar  mitoses  are 
also  common  in  regenerating  tissues  after  irritative  stimulus  (Strobe); 
but  it  is  uncertain  whether  such  mitoses  lead  to  the  formation  of 
normal  tissue.^ 

The  frequency  of  abnormal  mitoses  in  pathological  growths  is  a 
most  suggestive  fact,  but  it  is  still  wholly  undetermined  whether  the 
abnormal  mode  of  cell-division  is  the  cause  of  the  disease  or  the 
reverse.  The  latter  seems  the  more  probable  alternative,  since  normal 
mitosis    is  certainly  the  rule    in  abnormal    growths ;  and    Galeotti's 


^  The    remarkable    polyasters   formed    in 
scribed  at  p.  147. 


polyspermic   fertilization   of  the   egg  are  de- 


/O  CELL-DIVISION 

experiments  suggest  that  the  pathological  mitoses  in  such  growths 
may  be  caused  by  the  presence  of  deleterious  chemical  products  in 
the  diseased  tissue,  and  perhaps  point  the  way  to  their  medical 
treatment. 


D.     The  Mechanism  of  Mitosis 

We  now  pass  to  a  consideration  of  the  forces  at  work  in  mitotic 
division,  which  leads  us  into  one  of  the  most  debatable  fields  of 
cytological  inquiry. 

I.   Ficfictioji  of  the  Amphiaster 

All  observers  agree  that  the  amphiaster  is  in  some  manner  an 
expression  of  the  forces  by  which  cell-division  is  caused,  and  many 
accept,  in  one  form  or  another,  the  view  first  clearly  stated  by  YoX} 
that  the  asters  represent  in  some  manner  centres  of  attractive  forces 
focussed  in  the  centrosome  or  dynamic  centre  of  the  cell.  Regarding 
the  nature  of  these  forces,  there  is,  however,  so  wide  a  divergence  of 
opinion  as  to  compel  the  admission  that  we  have  thus  far  accom- 
plished little  more  than  to  clear  the  ground  for  a  precise  investigation 
of  the  subject;  and  the  mechanism  of  mitosis  still  lies  before  us  as 
one  of  the  most  fascinating  problems  of  cytology. 

{a)  The  Theoiy  of  Fibrillar  Contractility.  —  The  view  that  has 
taken  the  strongest  hold  on  recent  research  is  the  hypothesis  of 
fibrillar  contractility.  First  suggested  by  Klein  in  1878,  this  hypoth- 
esis was  independently  put  forward  by  Van  Beneden  in  1883,  and 
fully  outlined  by  him  four  years  later  in  the  following  words  :  "  In 
our  opinion,  all  the  internal  movements  that  accompany  cell-division 
have  their  immediate  cause  in  the  contractility  of  the  protoplasmic 
fibrillae  and  their  arrangement  in  a  kind  of  radial  muscular  system, 
composed  of  antagonizing  groups"  {i.e.  the  asters  with  their  rays). 
*'  In  this  system  the  central  corpuscle  (centrosome)  plays  the  part 
of  an  organ  of  insertion.  It  is  the  first  of  all  the  various  organs 
of  the  cells  to  divide,  and  its  division  leads  to  the  grouping  of  the 
contractile  elements  in  two  systems,  each  having  its  own  centre. 
The  presence  of  these  two  systems  brings  about  cell-division,  and 
actively  determines  the  paths  of  the  secondary  chromatic  asters " 
{i.e.  the  daughter-groups  of  chromosomes)  "  in  opposite  directions. 
An  important  part  of  the  phenomena  of  (karyo-)  kinesis  has  its  effi- 
cient cause,  not  in  the  nucleus,  but  in  the  protoplasmic  body  of  the 
cell."-      This  beautiful    hypothesis   was    based    on   very   convincing 

^  '73^  P-  473-  ^  '87^  P-  280. 


THE  MECHANISM   OF  MITOSIS 


71 


evidence  derived  from  the  study  of  the  Ascaris  o^gg,  and  it  was 
here  that  Van  Beneden  first  demonstrated  the  fact,  already  sus- 
pected by  Flemming,  that  the  daughter-chromosomes  move  apart  to 
the  pole§.  of  the  spindle, 
and  give  rise  to  the  two  re- 
spective daughter-nuclei. 1 

Van  Beneden  describes 
the  astral  rays,  both  in 
Ascaris  and  in  tunicates, 
as  differentiated  into  sev- 
eral groups  (Fig.  34).  One 
set,  forming  the  "  principal 
cone,"  are  attached  to  the 
chromosomes  and  form 
one-half  of  the  spindle, 
and,  by  the  contractions 
of  these  fibres,  the  chro- 
mosomes are  passively 
dragged  apart.  An  oppo- 
site group,  forming  the 
"  antipodal  cone,"  extend 
from  the  centrosome  to 
the  cell-periphery,  the  base 
of  the  cone  forming  the 
"polar  circle."  These 
rays,  opposing  the  action 
of  the  principal  cones,  not 
only  hold  the  centrosomes 
in  place,  but,  by  their  con- 
tractions, drag  them  apart, 
and  thus  cause  an  actual 
divergence  of  the  centres. 
The  remaining  astral  rays 
are  attached  to  the  cell- 
periphery  and  are  limited 
by  a  sub-equatorial  circle. 
Later  observations  indi- 
cate, however,  that  this 
arrangement  of  the  astral  rays  is  not  of  general  occurrence,  and  that 
the  rays  often  do  not  reach  the  periphery,  but  lose  themselves  in  the 
general  reticulum. 

Van  Beneden's  general  hypothesis  was  accepted  in  the  following 
year  by  Boveri  i^%'^,  2),  who  contributed  many  important  additional 

1  '83,  p.  544- 


CZ. 


m.z. 


Fig'  34'  —  Slightly  schematic  figures  of  dividing  eggs 
of  Ascaris,  illustrating  Van  Beneden's  theory  of  mitosis. 
[Van  Beneden  and  Julin.] 

A.  Early  anaphase ;  each  chromosome  has  divided 
into  two.  B.  Later  anaphase  during  divergence  of  the 
daughter-chromosomes,  a.c.  Antipodal  cone  of  astral 
rays ;  c.z.  cortical  zone  of  the  attraction-sphere ;  i.  in- 
terzonal fibres  stretching  between  the  daughter-chromo- 
somes ;  m.z.  medullary  zone  of  the  attraction-sphere; 
p.c.  principal  cone,  forming  one-half  of  the  contractile 
spindle  (the  action  of  these  fibres  is  reinforced  by  that  of 
the  antipodal  cone) ;  s.ex.  sub-equatorial  circle,  to  which 
the  astral  rays  are  attached. 


72 


CELL-DIVISION 


facts  in  its  support,  though  neither  his  observations  nor  those  of  later 
investigators  have  sustained  Van  Beneden's  account  of  the  grouping 
of  the  astral  rays.  Boveri  showed  in  the  clearest  manner  that,  during 
the  fertilization  of  Ascarts,  the  astral  rays  become  attached  to  the 
chromosomes  of  the  germ-nuclei ;  that  each  comes  into  connection  with 
rays  from  both  the  asters ;  that  the  chromosomes,  at  first  irregularly 
scattered  in  the  egg,  are  drawn  into  a  position  of  equilibrium  in  the 
equator  of  the  spindle  by  the  shortening  of  these  rays  (Figs.  65,  104); 
and  that  t/ie  rajs  tJiickcn  as  they  shorten.  He  showed  that  as  the 
chromosome  splits,  each  half  is  connected  only  with  rays  (spindle- 
fibres)  from  the  aster  on  its  own  side;  and  he  followed,  step  by  step, 


Fig.  35.  —  Leucocytes  or  wandering-cells  of  the  salamander,     [Heidenhain.] 
A.  Cell  with  a  single  nucleus  containing  a  very  coarse  network  of  chromatin  and  two  nucleoli 
(plasmosomes)  ;  s.  permanent  aster,  its  centre  occupied  by  a  double  centrosome  surrounded  by 
an  attraction-sphere.     B.  Similar  cell,  with  double  nucleus;  the  smaller  dark  masses  in  the  latter 
are  oxychromatin-granules  (linin),  the  larger  masses  are  basichromatin  (chromatin  proper). 

the  shortening  and  thickening  of  these  rays  as  the  daughter-chromo- 
somes diverge.  In  all  these  operations  the  behaviour  of  the  rays  is 
precisely  like  that  of  muscle-fibres  ;  and  it  is  difficult  to  study  Boveri's 
beautiful  figures  and  clear  descriptions  without  sharing  his  conviction 
that  "of  the  contractility  of  the  fibrilloe  there  can  be  no  doubt." ^ 

Very  convincing  evidence  in  the  same  direction  is  afforded  by 
pigment-cells  and  leucocytes  or  wandering-cells,  in  both  of  which 
there  is  a  very  large  permanent  aster  (attraction-sphere)  even  in  the 
resting-cell.  The  structure  of  the  aster  in  the  leucocyte,  where  it 
was  first  discovered  by  Flemming  in    1891,  has  been    studied   very 


2,  p.  99. 


THE  MECHANISM  OF  MITOSIS 


71 


carefully  by  Heidenhain  in  the  salamander.  The  astral  rays  here 
extend  throughout  nearly  the  whole  cell  (Fig.  35),  and  are  believed 
by  Heidenhain  to  represent  the  contractile  elements  by  means  of 
which  the. cell  changes  its  form  and  creeps  about.  A  similar  con- 
clusion was  reached  by  Solger  ('91)  and  Zimmerman  ('93,  2)  in  the 
case  of  pigment-cells  (chromatophores)  in  fishes.  These  cells  have, 
in  an  extraordinary  degree,  the  power  of  changing  their  form,  and  of 


Fig.  36.  —  Pigment-cells  and  asters  from  the  epidermis  of  fishes.     [Zimmerman.] 
A.  Entire  pigment-cell,  fiom  Blennius.     The  central  clear  space  is  the  central  mass  of  the  aster 
from  which  radiate  the  pigment-granules;  two  nuclei  below.     B.  Nucleus  («)  and  aster  after  ex- 
traction of  the  pigment,  showing  reticulated  central  mass.      C.  Two  nuclei  and  aster  with  rod- 
shaped  central  mass,  from  Sargus.- 


actively  creeping  about.  Solger  and  Zimmerman  have  shown  that 
the  pigment-cell  contains  an  enormous  aster,  whose  rays  extend  in 
every  direction  through  the  pigment-mass,  and  it  is  almost  impos- 
sible to  doubt  that  the  aster  is  a  contractile  apparatus,  like  a  radial 
muscular  system,  by  means  of  which  the  active  changes  of  form  are 
produced  (Fig.  36). 

But  although  these  observations  seem  to  place  the  theory  of  fibrillar 
contractility  upon  a  firm  basis,  it  has  since  undergone  various   modifi- 


74  CELL-DIVISION 

cations  and  limitations,  which  show  that  the  matter  is  by  no  means 
so  simple  as  it  first  appeared.  The  most  important  of  these  modifi- 
cations are  due  to  Hermann  ('91)  and  Driiner  ('95),  who  have  relied 
mainly  on  the  study  of  mitosis  in  various  cells  of  the  salamander,  well 
known  as  extremely  favourable  objects  for  study.  These  observers 
have  demonstrated  that  in  this  case  the  spindle-fibres  are  of  two 
kinds  which,  apparently,  differ  both  in  origin  and  in  mode  of  action. 
Hermann  showed  that  the  primary  amphiaster  is  formed  outside  the 
nucleus,  without  connection  with  the  chromosomes,  and  that  the 
original  spindle  persists  as  a  "central  spindle"  (Figs.  21,  22),  which 
he  regards  as  composed  of  non-contractile  fibres,  and  merely  forming 
a  support  on  which  the  movements  of  the  chromosomes  take  place. 
The  contractile  elements  are  formed  by  certain  of  the  astral  rays 
which  grow  into  the  nucleus,  and  become  attached  to  the  chromo- 
somes, as  Boveri  described.  By  the  contraction  of  these  latter  fibres 
the  chromosomes  are  now  dragged  towards  the  spindle,  and  around 
its  equator  they  are  finally  grouped  to  form  the  equatorial  plate.  The 
fully  formed  spindle  consists,  therefore,  of  two  elements ;  namely,  {a) 
the  original  "  central  spindle,"  and  {b)  a  surrounding  mantle  of  con- 
tractile "  mantle-fibres  "  attached  to  the  chromosomes,  and  originally 
derived  from  astral  rays.  In  the  anaphase,  as  Hermann  believes,  the 
danghter-cJiromosonies  are  dragged  apart  solely  by  the  contractile  mantle- 
fibres,  the  central  spindle-fibj'es  beijig  non-contractile  and  servitig  as  a 
support  or  substratnin  along  which  the  chromosomes  move.  As  the 
chromosomes  diverge,  the  central  spindle  comes  into  view  as  the  in- 
terzonal fibres  (Fig.  22,  G,  H).  Strasburger  ('95)  is  now  inclined  to 
accept  a  similar  view  of  mitosis  in  the  cells  of  plants. 

Driiner  ('95)  in  his  beautiful  studies  on  the  mechanism  of  mitosis 
has  advanced  a  step  beyond  Hermann,  maintaining  that  the  pro- 
gressive divergence  of  the  spindle-poles  is  caused  by  an  active 
growth  or  elongation  of  the  central  spindle  which  goes  on  throughout 
the  whole  period  from  the  earliest  prophases  until  the  close  of  the 
anaphases.  This  view  is  supported  by  the  fact  that  the  central 
spindle-fibres  are  always  contorted  during  the  metaphases,  as  if 
pushing  against  a  resistance;  and,  as  Richard  Hertwig  points  out 
('95),  it  harmonizes  with  the  facts  observed  in  the  mitoses  of  in- 
fusorian  nuclei.  The  same  view  is  adopted  by  Braus  and  by 
Reinke.  Flemming  ('95)  is  still  inclined,  however,  to  the  view  that 
the  divergence  of  the  centres  may  be  in  part  caused  by  the  trac- 
tion of  the  antipodal  fibres,  as  maintained  by  Van  Beneden  and 
Boveri. 

Heidenhain,  finally,  while  accepting  the  contractility-hypothesis, 
ascribes  only  a  subordinate  role  to  an  active  physiological  contrac- 
tility of  the  fibres.     The  main  factor  in  mitosis  is  ascribed  to  clastic 


THE   MECHANISM  OF  MITOSIS  75 

tension  of  the  astral  rays  which  are  attached  at  one  end  to  the  cen- 
trosome,  at  the  other  to  the  cell-periphery.  By  turgor  of  the  cell 
the  rays  are  passively  stretched,  thus  causing  divergence  of  the 
spindle-polps  and  of  the  daughter-chromosomes  to  which  the  spin- 
dle-fibres are  attached.  An  active  contraction  of  the  fibres  is  only 
invoked  to  explain  the  closing  phases  of  mitosis. 

{b)  Other  Theories. — Watase's  ingenious  theory  of  mitosis  ('93) 
is  exactly  the  opposite  of  Van  Beneden's,  assuming  that  the  spindle- 
fibres  are  not  pulling  but  pushing  agents,  the  daughter-chromo- 
somes being  forced  apart  by  continually  lengthening  fibres  which 
grow  out  from  the  centres  and  dovetail  in  the  region  of  the  inter- 
zonal fibres.  Each  daughter-chromosome  is  therefore  connected 
with  fibres  from  the  aster,  not  of  its  own,  but  of  the  opposite 
side.  This  view  is,  I  believe,  irreconcilable  with  the  movements 
of  chromosomes  observed  in  multiple  asters,  and  also  with  those 
that  occur  during  the  fertilization  of  the  ^g^g,  where  the  chromo- 
somes are  plainly  drawn  towards  the  astral  centres  and  not  pushed 
away  from  them. 

Blitschli,  Carnoy,  Platner,  and  others  have  sought  an  explanation 
in  a  totally  different  direction  from  any  of  the  foregoing,  regarding 
the  formation  of  the  amphiaster  as  due  essentially  to  streaming  or 
osmotic  movements  of  the  fluid  constituents  of  the  protoplasm,  and 
the  movements  of  the  chromosomes  as  being  in  a  measure  mechan- 
ically caused  by  the  same  agency.  Oscar  Hertwig  adopts  a  some- 
what vague  dynamical  view,  regarding  the  formation  of  the  mitotic 
figure  as  due  to  an  interaction  between  nucleus  and  cytoplasm,  which 
he  compares  to  that  taking  place  in  a  magnetic  field  between  a  mag- 
net and  a  mass  of  iron  filings  :  "  The  interaction  between  nucleus  and 
protoplasm  in  the  cell  finds  its  visible  expression  in  the  formation  of 
the  polar  centres  and  astral  figures ;  the  result  of  the  interaction  is 
that  the  nucleus  always  seeks  the  middle  of  its  sphere  of  action."  ^ 
He  gives,  however,  no  hint  of  his  view  regarding  the  nature  of  the 
action  or  the  causes  of  the  chromosomal  movements.  Ziegler  ('95) 
accepts  a  somewhat  similar  view ;  and  he  has  shown  that  surpris- 
ingly close  simulacra  of  the  mitotic  figure  in  many  of  its  different 
phases  may  be  produced  by  placing  bent  wires  (representing  the 
chromosomes)  in  the  field  of  a  horseshoe  magnet  strewn  with  iron 
filings. 

My  own  studies  on  the  eggs  of  echinoderms  ('95,  2)  and  annelids 
have  convinced  me  that  no  adequate  hypothesis  of  the  mitotic  mech- 
anism has  yet  been  advanced.  In  these,  as  in  many  other  forms,  the 
spindle-fibres  show  no  differentiation  into  central   spindle  and  peri- 

^  Zelle  und  Gewebe,  p.  172. 


76 


CELL-DIVISION 


pheral  mable-fibres ;  and  the  chromosomes  extend  entirely  through 
the  substance  of  the  spindle  in  its  equatorial  plane.  If  there  be  sup- 
porting, as  opposed  to  contractile,  fibres,  they  must  be  intermingled 
with  the  latter ;  and  both  forms  must  have  the  same  origin.     The 


\^\' 


\  \.\.;v,.;,////. , 


^^Z-  37-  — The  later  stages  of  mitosis  in  the  egg  of  the  sea-urchin  Toxoptieiistes  {A-D,  X  looo; 
E-F,  X  500). 

A.  Metaphase  ;  daughter-chromosomes  drawing  apart  but  still  united  at  one  end.  B.  Daugh- 
ter-chromosomes separating.  C,  Late  anaphase ;  daughter-chromosomes  lying  at  the  spindle- 
poles.  D.  Final  annphase;  daughtttr-chromosomes  converted  into  vesicles,  E.  Immediately 
after  division,  the  asters  undivided;  the  spindle  has  disappeared.  F.  Resting  2-ceIl  stage,  the 
asters  divided  into  two  in  anticipation  of  the  next  division. 

In  Figs.  A  to  D,  the  centrosphere  appears  as  a  large  reticulated  mass  from  which  the  rays  pro- 
ceed. It  is  probable  that  a  minute  centrosome,  or  pair  of  centrosomes,  lies  near  the  centre  of  the 
centrosphere,  but  this  is  not  shown. 


daughter-chromosomes  appear  to  move  towards  the  poles  through 
the  substance  of  the  spindle,  and  do  not  travel  along  its  periphery  as 
described  by  Hermann  and  Driiner  in  amphibia  and  by  Strasburger 
('93,  2)  in  the  plants.      No  shortening  or  thickening  of  the  ravs  can 


THE  MECHANISM   OF  MITOSIS  77 

be  observed,  and  the  chromosomes  proceed  to  the  extreme  limit  of 
the  spindle-poles  and  appear  actually  to  pass  into  the  interior  of  the 
huge  reticulated  centrosphere.  I  cannot  see  how  this  behaviour  of 
the  chromosomes  is  to  be  explained  as  the  result  solely  of  a  con- 
traction of  fibres  stretching  between  them  and  the  centrosphere. 
It  is  certain,  moreover,  that  another  factor  is  at  work.  Throughout 
the  anaphases,  the  centrosphere  steadily  grows  until,  at  the  close, 
it  attains  an  enormous  size  (Fig.  37),  and  its  substance  differs  chem- 
ically from  that  of  the  rays,  for  after  double  staining  with  Congo 
red  (an  acid  aniline)  and  haematoxylin  it  becomes  bright  red  while 
the  rays  are  blue.  It  seems  probable,  therefore,  that  the  movements 
of  the  chromosomes  are  affected  by  definite  chemical  changes  occur- 
ring in  the  centrosphere,  as  Biitschli^  and  Strasburger^  have  main- 
tained ;  and  it  is  possible*  that  the  substance  of  the  spindle-fibres 
may  be  actually  taken  up  into  the  centrosphere,  and  the  chromo- 
somes thus  drawn  towards  it.  Strasburger  has  made  the  interesting 
suggestion,  which  seems  well  worthy  of  consideration,  that  the  move- 
ments of  the  chromosomes  may  be  of  a  chemotactic  character.  In 
any  case,  I  believe  that  no  satisfactory  hypothesis  can  be  framed 
that  does  not  reckon  with  the  chemical  and  physical  changes  going 
on  in  the  centrosphere,  and  take  into  account  also  the  probability 
of  a  dynamic  action  radiating  from  it  into  the  surrounding  struct- 
ures. Van  Beneden's  hypothesis  is  probably,  in  principle,  correct ; 
but,  as  Boveri  himself  admits  in  his  latest  paper  ('95),  it  seems  cer- 
tain that  other  factors  are  involved  besides  the  contractility  of  the 
achromatic  fibres,  and  the  mechanism  of  mitosis  still  awaits  adequate 
physiological  analysis. 

2.    Divisio7i  of  the  Chromosomes 

In  developing  his  theory  of  fibrillar  contractility  Van  Beneden 
expressed  the  view  —  only,  however,  as  a  possibility  —  that  the 
splitting  of  the  chromosomes  might  be  passively  caused  by  the  con- 
tractions of  the  two  sets  of  opposing  spindle-fibres  to  which  each  is 
attached.^  Later  observations  have  demonstrated  that  this  sugges- 
tion cannot  be  sustained ;  for  in  many  cases  the  chromatin-thread 
splits  before  division  of  the  centrosome  and  the  formation  of  the 
achromatic  figure,  —  sometimes  during  the  spireme-stage,  or  even  in 
the  reticulum,  while  the  nuclear  membrane  is  still  intact.  Boveri 
showed  this  to  be  the  case  in  Ascai'is,  and  a  similar  fact  has  been 
observed   by  many  observers   since,  both  in  plants   and  in  animals. 

The  splitting  of  the  chromosomes  is  therefore,  in  Boveri's  words, 

1  '92,  pp.  158,  159.  ^  '93,  2.  ■'  '87,  p.  279. 


78 


CELL-DIVISION 


''ail  independent  vital. manifestation,  an  act  of  reprodnction  on  tJie  part 
of  the  chromosomes.''  ^ 

All  of  the  recent  researches  in  this  field  point  to  the  conclusion 
that  this  act  of  division  must  be  referred  to  the  fission  of  the 
chromatin-granules  or  chromomeres  of  which  the  chromatin-thread 
is  built.  These  granules  were  first  clearly  described  by  Balbiani 
('76)  in  the  chromatin-network  of  epithelial  cells  in  the  insect- 
ovary,  and  he  found  that  the  spireme-thread  arose  by  the  linear 
arrangement  of  these  granules  in  a  single  row  like  a  chain  of  bacte- 
ria.2  Six  years  later  Pfitzner  ('72)  added  the  interesting  discovery, 
that  during  the  mitosis  of  various  tissue-cells  of  the  salamander,  the 
granules  of  the  spireme-thread  divide  by  fission  and  tJuis  determine  the 


Fig.  38.  —  Nuclei  in  the  spireme-sfage. 

A.  From  the  endosperm  of  the  lily,  showing  true  nucleoli.     [Flemming.] 

B.  Spermatocyte  of  salamander.      Segmented  double  spireme-thread  composed  of  chromo- 
meres and  completely  split.    Two  centrosomes  and  central  spindle  at  s.     [Hermann.] 

C.  Spireme-thread  completely  split,  with  six  nucleoli.     Endosperm   of  Fritillaria.     [FLEM- 
MING.] 

longitudinal  splitting  of  the  entire  chromosome.  This  discovery  was 
confirmed  by  Flemming  in  the  following  year  ('82,  p.  219),  and  a  sim- 
ilar result  has  been  reached  by  many  other  observers  (Fig.  38).  The 
division  of  the  chromatin-granules  may  take  place  at  a  very  early 
period.  Flemming  observed  as  long  ago  as  1881  that  the  chromatin- 
thread  might  split  in  the  spireme-stage  (epithelial  cells  of  the  sala- 
mander), and  this  has  since  been  shown  to  occur  in  many  other  cases; 
for  instance,  by  Guignard  in  the  mother-cells  of  the  pollen  in  the 
lily  ('91).  Brauer's  recent  work  on  the  spermatogenesis  of  Ascaris 
shows  that  the  fission  of  the  chromatin-granules  here  takes  place  even 
before  the  spireme-stage,  when  the  chromatin  is  still  in  the  form  of  a 


p.  113. 


2  See  '81,  p.  638. 


THE  MECHANISM   OF  MITOSIS 


79 


reticulum,  and  long  before  the  division  of  the  centrosome  (Fig.  39). 
He  therefore  concludes:  "With  Boveri  I  regard  the  splitting  as  an 
independent  reproductive  act  of  the  chromatin.  The  reconstruction 
of  the  nucjeus,  and  in  particular  the  breaking  up  of  the  chromosomes 
after  division  into  small  granules  and  their  uniform  distribution 
through  the  nuclear  cavity,  is,  in  the  first  place,  for  the  purpose  of 


11       w^/ 


F 


Fig.  39.  —  Formation  of  chromosomes  and  early  splitting  of  the  chromatin-granules  in  sperma- 
togonia of  Ascaris  7negalocephala,  var.  bivalens.     [Brauer.] 

A.  Very  early"  prophase ;  granules  of  the  nuclear  reticulum  already  divided.  B.  Spireme ; 
the  continuous  chromatin-thread  split  throughout.  C.  Later  spireme.  D.  Shortening  of  the 
thread.  E.  Spireme-thread  divided  into  two  parts.  F.  Spireme-thread  segmented  into  four  split 
chromosomes. 


allowing  a  uniform  growth  to  take  place ;  and  in  the  second  place, 
after  the  granules  have  grown  to  their  normal  size,  to  admit  of  their 
precisely  equal  quantitative  and  qualitative  division.  I  hold  that  all 
the  succeeding  phenomena,  such  as  the  grouping  of  the  granules 
in  threads,  their  union  to  form  larger  granules,  the  division  of  the 
thread  into  segments  and  finally  into  chromosomes,  are  of  secondary 
importance;  all  these  are  only  for  the  purpose  of  bringing  about  in 


8o  CELL-DIVISION 

the  simplest  and  most  certain  manner,  the  transmission  of  the  daugh- 
ter-granules (Spalthatften)  to  the  daughter-cells."^  *' In  my  opinion 
the  chromosomes  are  not  independent  individuals,  but  only  groups  of 
numberless  minute  chromatin-granules,  which  alone  have  the  value 
of  individuals."  2 

These  observations  certainly  lend  strong  support  to  the  view  that 
the  chromatin  is  to  be  regarded  as  a  morphological  aggregate  —  as 
a  congeries  or  colony  of  self-propagating  elementary  organisms 
capable  of-  assimilation,  growth,  and  division.  They  prove,  more- 
over, that  mitosis  involves  two  distinct  though  closely  related  factors, 
one  of  which  is  the  fission  of  the  chromatic  nuclear  substance,  while 
the  other  is  the  distribution  of  that  substance  to  the  daughter-cells. 
In  the  first  of  these  it  is  the  chromatin  that  takes  the  active  part ; 
in  the  second  it  would  seem  that  the  main  role  is  played  by  the 
archoplasm,  or  in  the  last  analysis,  the  centrosome. 


E.     Direct  or  Amitotic  Division 
I.    General  Sketch 

We  turn  now  to  the  rarer  and  simpler  mode  of  division  known 
as  amitosis ;  but  as  Flemming  has  well  said,  it  is  a  somewhat  trying 
task  to  give  an  account  of  a  subject  of  which  the  final  outcome  is 
so  unsatisfactory  as  this;  for  in  spite  of  extensive  investigation,  we 
still  have  no  very  definite  conclusion  in  regard  either  to  the  mechan- 
ism of  amitosis  or  its  biological  meaning.  Amitosis,  or  direct  division, 
differs  in  two  essential  respects  from  mitosis.  First,  the  nucleus 
remains  in  the  resting  state  (reticulum),  and  there  is  no  formation 
of  a  spireme  or  of  chromosomes.  Second,  division  occurs  without 
the  formation  of  an  amphiaster;  hence  the  centrosome  is  not  con- 
cerned with  the  nuclear  division,  which  takes  place  by  a  simple 
constriction.  The  nuclear  substance,  accordingly,  undergoes  a  divi- 
sion of  its  total  mass,  but  not  of  its  individual  elements  or  chromatin- 
granules  (Fig.  40). 

Before  the  discovery  of  mitosis,  nuclear  division  was  generally 
assumed  to  take  place  in  accordance  with  Remak's  scheme  (p.  45). 
The  rapid  extension  of  our  knowledge  of  mitotic  division  between 
the  years  1875  and  1885  showed,  however,  that  such  a  mode  of 
division  was,  to  say  the  least,  of  rare  occurrence,  and  led  to  doubts 
as  to  whether  it  ever  actually  took  place  as  a  normal  process.  As 
soon,  however,  as   attention  was  especially  directed  to  the  subject, 

2  i_c,^  p.  205. 


DIRECT   OR  AMITOTIC  DIVISION 


8i 


many  cases  of  amitotic  division  were  accurately  determined,  though 
very  few  of  them  conformed  precisely  to  Remak's  scheme.  One 
such  case  is  that  described  by  Carnoy  in  the  follicle-cells  of  the 
(tg^  in  the  mole-cricket,  where  division  begins  in  the  fission  of  the 
nucleolus,    followed   by   that   of   the   nucleus.     Similar   cases    have 


Fig.  40.  —  Group  of  cells  with  amitotically  dividing  nuclei ;  ovarian  follicular  epithelium  of 
the  cockroach.     [Wheeler.] 

been  since  described,  by  Hoyer  ('90)  in  the  intestinal  epithelium  of 
the  nematode  Rhabdonemay  by  Korschelt  in  the  intestine  of  the 
annelid  Ophryotrocha,  and  in  a  few  other  cases.  In  many  cases,  how- 
ever, no  preliminary  fission  of  the  nucleolus  occurs ;  and  Remak's 
scheme  must,  therefore,  be  regarded  as  one  of  the  rarest  forms  of 
cell-division  ( ! ). 


2.    Centrosome  and  Attraction- Sphere  in  Amitosis 

The  behaviour  of  the  centrosome  in  amitosis  forms  an  interesting  question 
on  account  of  its  bearing  on  the  mechanics  of  cell-division.  Flemming  ob- 
served ('91)  that  the  nucleus  of  leucocytes  might  in  some  cases  divide  directly 
without  the  formation  of  an  amphiaster,  the  attraction-sphere  remaining  undivided 
meanwhile.  Heidenhain  showed  in  the  following  year,  however,  that  in  some 
cases  leucocytes  containing  two  nuclei  (doubtless  formed  by  amitotic  division) 
might  also  contain  two  asters  connected  by  a  spindle.  Both  Heidenhain  and 
Flemming  drew  from  this  the  conclusion  that  direct  division  of  the  nucleus  is  in 
this  case  independent  of  the  centrosome,  but  that  the  latter  might  be  concerned  in 
the  division  of  the  cell-body,  though  no  such  process  was  observed.  A  little  later, 
however,  Meves  published  remarkable  observations  that  seem  to  indicate  a  functional 
activity  of  the  attraction-sphere  during  amitotic  nuclear  division  in  the  "sperma- 


82  CELL-DIVISION 

togonia  "  of  the  salamander. ^  Krause  and  Flemming  observed  that  in  the  autumn 
many  of  these  cells  show  peculiarly-lobed  and  irregular  nuclei  (the  "  polymorphic 
nuclei"  of  Bellonci).  These  were,  and  still  are  by  some  writers,  regarded  as 
degenerating  nuclei.  Meves,  however,  asserts  —  and  the  accuracy  of  his  obser- 
vations is  in  the  main  vouched  for  by  Flemming  —  that  in  the  ensuing  spring 
these  nuclei  become  uniformly  rounded,  and  may  then  divide  amitotically.  In 
the  autumn  the  attraction-sphere  is  represented  by  a  diffused  and  irregular  granu- 
lar mass,  which  more  or  less  completely  surrounds  the  nucleus.  In  the  spring,  as 
the  nuclei  become  rounded,  the  granular  substance  draws  together  to  form  a  definite 
rounded  sphere,  in  which,  a  distinct  centrosome  may  sometimes  be  made  out. 
Division  takes  place  in  the  following  extraordinary  manner :  The  nucleus  assumes 
a  dumb-bell  shape,  while  the  attraction-sphere  becomes  drawn  out  into  a  band 
which  surrounds  the  central  part  of  the  nucleus,  and  finally  forms  a  closed  ring, 
encircling  the  nucleus.  After  this  the  nucleus  divides  into  two,  while  the  ring- 
shaped  attraction-sphere  ("  archoplasm ")  is  again  condensed  into  a  sphere.  The 
appearances  suggest  that  the  ring-shaped  sphere  actually  compresses  the  nucleus, 
and  cuts  it  through.  In  a  later  paper  ('94),  Meves  shows  that  the  diffused  ''archo- 
plasm"  of  the  autumn-stage  arises  by  the  breaking  down  of  a  definite  spherical 
attraction-sphere,  which  is  reformed  again  in  the  spring  in  the  manner  described, 
and  in  this  condition  the  cells  may  divide  either  initotically  or  amitotically .  He 
adds  the  interesting  observation,  since  confirmed  by  Rawitz  ('94),  that  in  the 
spermatocytes  of  the  salamander,  the  attraction-spheres  of  adjoining  cells  are  often 
connected  by  intercellular  bridges,  but  the  meaning  of  this  has  not  yet  been 
determined. 

It  is  certain  that  the  remarkable  transformation  of  the  sphere  into  a  ring  during 
amitosis  is  not  of  universal,  or  even  of  general,  occurrence,  as  shown  by  the  later 
studies  of  vom  Rath  ('95,  3).  In  leucocytes,  for  example,  the  sphere  persists  in 
its  typical  form,  and  contains  a  centrosome,  during  every  stage  of  the  division ;  but 
it  is  an  interesting  fact  that  during  all  these  stages  the  sphere  lies  on  the  concave 
side  of  the  nucleus  in  the  bay  which  finally  cuts  through  the  entire  nucleus.  Again, 
in  the  liver-cells  of  the  isopod  Porcellio,  the  nucleus  divides,  not  by  constriction,  as 
in  the  leucocyte,  but  by  the  appearance  of  a  nuclear  plate,  in  the  formation  of  which 
the  attraction-sphere  is  apparently  not  concerned. ^  The  relations  of  the  centro- 
some and  archoplasm  in  amitosis  are,  therefore,  still  in  doubt ;  but,  on  the  whole, 
the  evidence  goes  to  show  that  they  take  no  essential  part  in  the  process. 


3.    Biological  Signijicance  of  Amitosis 

A  survey  of  the  known  cases  of  amitosis  brings  out  the  following 
significant  facts.  It  is  of  extreme  rarity,  if  indeed  it  ever  occurs  in 
embryonic  cells  or  such  as  are  in  the  course  of  rapid  and  continued 
multiplication.  It  is  frequent  in  pathological  growths  and  in  cells 
such  as  those  of  the  vertebrate  decidua,  of  the  embryonic  envelopes 
of  insects,  or  the  yolk-nuclei  (periblast,  etc.),  za/iic/z  are  on  the  way 
towai'ds  degeneration.  In  many  cases,  moreover,  direct  nuclear  divi- 
sion is  not  followed  by  fission  of  the  cell-body,  so  that  multinuclear 

1  '91,  p.  628. 

2  Such  a  mode  of  amitotic  division  was  first  described  by  Sabatier  in  the  Crustacea  ('89), 
and  a  similar  mode  has  been  observed  by  Carnoy  and  Van  der  Stricht. 


DIRECT   OR  AMITOTIC  DIVISION  83 

cells  and  polymorphic  nuclei  are  thus  often  formed.  These  and 
many  similar  facts  led  Flemming  in  1891  to  express  the  opinion  that 
so  far  as  the  higher  plants  and  animals  are  concerned  amitosis  is  "  a 
process  w];^ich  does  not  lead  to  a  new  production  and  multiplication 
of  cells,  but  wherever  it  occurs  represents  either  a  degeneration  or  an 
aberration,  or  perhaps  in  many  cases  (as  in  the  formation  of  multi- 
nucleated cells  by  fragmentation)  is  tributary  to  metabolism  through 
the  increase  of  nuclear  surface."  ^  In  this  direction  Flemming 
sought  an  explanation  of  the  fact  that  leucocytes  may  divide  either 
mitotically  or  amitotically  (t.  Peremeschko,  Lowit,  Arnold,  Flemming). 
In  the  normal  lymph-glands,  where  new  leucocytes  are  continually 
regenerated,  mitosis  is  the  prevalent  mode.  Elsewhere  (wandering- 
cells)  both  processes  occur.  "  Like  the  cells  of  other  tissues  the 
leucocytes  find  their  normal  physiological  origin  (Neubildung)  in 
mitosis ;  only  those  so  produced  have  the  power  to  live  on  and  repro- 
duce their  kind  through  the  same  process."^  Those  that  divide  ami- 
totically are  on  the  road  to  ruin.  Amitosis  in  the  higher  forms  is 
thus  conceived  as  a  purely  secondary  process,  not  a  survival  of  a 
primitive  process  of  direct  division  from  the  Protozoa,  as  Strasburger 
('82)  and  Waldeyer  {'^^^  had  conceived  it. 

This  hypothesis  has  been  carried  still  further  by  Ziegler  and  vom 
Rath  ('91).  In  a  paper  on  the  origin  of  the  blood  in  fishes,  Ziegler 
{^'^J^  showed  that  the  periblast-nuclei  in  the  eggs  of  fishes  divide 
amitotically,  and  he  was  thus  led  like  Flemming  to  the  view  that 
amitosis  is  connected  with  a  high  specialization  of  the  cell  and  may 
be  a  forerunner  of  degeneration.  In  a  second  paper  ('91),  published 
shortly  after  Flemming's,  he  points  out  the  fact  that  amitotically 
dividing  nuclei  are  usually  of  large  size  and  that  the  cells  are  in 
many  cases  distinguished  by  a  specially  intense  secretory  or  assimi- 
lative activity.  Thus,  Riige  ('90)  showed  that  the  absorption  of 
degenerate  eggs  in  the  amphibia  is  effected  by  means  of  leuco- 
cytes which  creep  into  the  egg-substance.  The  nuclei  of  these 
cells  become  enlarged,  divide  amitotically,  and  then  frequently 
degenerate.  Other  observers  (Korschelt,  Carnoy)  have  noted  the 
large  size  and  amitotic  division  of  the  nuclei  in  the  ovarian  follicle- 
cells  and  nutritive-cells  surrounding  the  ovum  in  insects  and  Crusta- 
cea. Chun  found  in  the  entodermic  cells  of  the  radial  canals  of 
Siphonophores  huge  cells  filled  with  nests  of  nuclei  amitotically 
produced,  and  suggested  ('90)  that  the  multiplication  of  nuclei  was 
for  the  purpose  of  increasing  the  nuclear  surface  as  an  aid  to 
metabolic  interchanges  between  nucleus  and  cytoplasm.  Amitotic 
division  leading  to  the  formation   of  multinuclear  cells  is  especially 

1  '91,  2,  p.  291. 


84  CELL-DIVISION 

common  in  gland-cells.  Thus,  Klein  has  described  such  divisions  in 
the  mucous  skin-glands  of  Amphibia,  and  more  recently  vom  Rath 
has  carefully  described  it  in  the  huge  gland-cells  (probably  salivary) 
of  the  isopod  Anilocm  ('95).  Many  other  cases  are  known.  Dogiel 
('90)  has  observed  exceedingly  significant  facts  in  this  field  that  place 
the  relations  between  mitosis  and  amitosis  in  a  clear  light.  It  is  a 
well-known  fact  that  in  stratified  epithelium,  new  cells  are  continually 
formed  in  the  deeper  layers  to  replace  those  cast  off  from  the 
superficial  layers.  Dogiel  finds  in  the  lining  of  the  bladder  of  the 
mouse  that  the  nuclei  of  the  superficial  cells,  which  secrete  the  mucus 
covering  the  surface,  regularly  divide  amitotically,  giving  rise  to  huge 
multinuclear  cells,  which  finally  degenerate  and  are  cast  off.  The 
new  cells  that  take  their  place  are  formed  in  the  deeper  layers  by 
mitosis  alone.  Especially  significant,  again,  is  the  case  of  the  ciliate 
Infusoria,  which  possess  two  kinds  of  nuclei  in  the  same  cell,  a 
macronucleus  and  a  micronucleus.  The  former  is  known  to  be 
intimately  concerned  with  the  processes  of  metabolism  (cf.  p.  165). 
During  conjugation  the  macronucleus  degenerates  and  disappears 
and  a  new  one  is  formed  from  the  micronucleus  or  one  of  its 
descendants.  The  macronucleus  is  therefore  essentially  metabolic, 
the  micronucleus  generative  in  function.  In  view  of  this  contrast  it 
is  a  significant  fact  that  while  both  nuclei  divide  during  the  ordinary 
process  of  fission  the  mitotic  phenomena  are  as  a  rule  less  clearly 
marked  in  the  macronucleus  than  in  the  micronucleus,  and  in  some 
cases  the  former  appears  to  divide  directly  while  the  latter  always 
goes  through  a  process  of  mitosis.  In  view  of  all  these  facts  and 
others  of  like  import  Ziegler,  like  Flemming,  concludes  that  amitosis 
is  of  a  secondary  character,  and  that  when  it  occurs  the  series  of 
divisions  is  approaching  an  end. 

This  conclusion  received  a  very  important  support  in  the  work  of 
vom  Rath  on  amitosis  in  the  testis  ('93).  On  the  basis  of  a  compara- 
tive study  of  amitosis  in  the  testis-cells  of  vertebrates,  mollusks,  and 
arthropods  he  concludes  that  amitosis  never  occurs  in  the  sperm- 
producing  cells  (spermatogonia,  etc.),  but  only  in  the  supporting  cells 
(Randzellen,  Stiitzzellen).  The  former  multiply  through  mitosis 
alone.  The  two  kinds  of  cells  have,  it  is  true,  a  common  origin  in 
cells  which  divide  mitotically.  When,  however,  they  have  once 
become  differentiated,  they  remain  absolutely  distinct;  amitosis 
never  takes  place  in  the  series  which  finally  results  in  the  formation 
of  spermatozoa,  and  .the  amitotically  dividing  "  supporting-cells " 
sooner  or  later  perish.  Vom  Rath  thus  reached  the  remarkable  con- 
clusion that  ""  when  once  a  cell  has  undergone  amitotic  division  it 
has  received  its  death-warrant ;  it  may  indeed  continue  for  a  time  to 
divide  by  amitosis,  but  inevitably  perishes  in  the  end."     ('91,  p.  331.) 


SUMMARY  AND    CONCLUSION  85 

Whether  this  conclusion  can  be  accepted  without  modification 
remains  to  be  seen.  Flemming  himself  regards  it  as  too  extreme, 
and  is  inclined  to  accept  Meves'  conclusion  that  amitosis  may  occur 
in  the  sp^rm-producing  cells  of  the  testis.  The  same  conclusion  is 
reached  by  Preusse  in  the  case  of  insect-ovaries.  There  can  be  no 
doubt,  however,  that  Flemming's  hypothesis  in  a  general  way  repre- 
sents the  truth,  and  that  in  the  vast  majority  of  cases  amitosis  is 
a  secondary  process  which  does  not  fall  in  the  generative  series  of 
cell-divisions. 


F.     Summary  and  Conclusion 

Three  distinct  elements  are  involved  in  the  typical  mode  of  cell- 
division  by  mitosis ;  namely,  the  centrosome,  the  chromosome,  and  the 
cell-body.  Of  these,  the  centrosome  may  be  considered  the  organ 
of  division  par  excelletice ;  for  as  a  rule  it  leads  the  way  in  division, 
and  under  its  influence,  in  some  unknown  manner,  is  organized  the 
astral  system  which  is  the  immediate  instrument  of  division.  This 
system  appears  in  the  form  of  two  asters,  each  containing  one  of  the 
daughter-centrosomes  and  connected  by  a  spindle  to  form  an  ampJii- 
aster.  It  arises  as  a  differentiation  or  morphological  rearrangement 
of  the  general  cell-reticulum,  the  asters  being  formed  from  the  extra- 
nuclear  reticulum,  the  spindle  sometimes  from  the  linin-network, 
sometimes  from  the  cyto-reticulum,  sometimes  from  both. 

The  chromosomes,  always  of  the  same  number  in  a  given  species 
(with  only  apparent  exceptions),  arise  by  the  transformation  of  the 
chromatin-reticulum  into  a  thread  which  breaks  into  segments  and 
splits  lengthwise  throughout  its  whole  extent.  The  two  halves  are 
thereupon  transported  in  opposite  directions  along  the  spindle  to 
its  respective  poles  and  there  enter  into  the  formation  of  the  two 
corresponding  daughter-nuclei.  The  spireme-thread,  and  hence  the 
chromosome,  is  formed  as  a  single  series  of  chromatin-granules  or 
chromomeres  which,  by  their  fission,  cause  the  splitting  of  the  thread. 
Every  individual  chromatin-granule  therefore  contributes  its  quota 
to  each  of  the  daughter-nuclei. 

The  mechanism  of  mitosis  is  imperfectly  understood.  There  is 
good  reason  to  believe  that  the  fission  of  the  chromatin-granules,  and 
hence  the  splitting  of  the  thread,  is  not  caused  by  division  of  the 
centrosome,  but  only  accompanies  it  as  a  parallel  phenomenon.  The 
divergence  of  the  daughter-chromosomes,  on  the  other  hand,  is  in 
some  manner  determined  by  the  spindle-fibres  developed  under  the 
influence  of  the  centrosomes.  There  are  cogent  reasons  for  the  view 
that  some  at  least  of  these  fibres  are' contractile  elements  which,  like 


86  CELL-DIVISION 

muscle-fibres,  drag  the  daughter-chromosomes  asunder;  while  other 
spindle-fibres  act  as  supporting  and  guiding  elements,  and  probably 
by  their  elongation  push  the  spindle-poles  apart.  The  contractility 
hypothesis  is,  however,  difficult  to  apply  in  certain  cases,  and  is  prob- 
ably an  incomplete  explanation  which  awaits  further  investigation. 
The  functions  of  the  astral  rays  are  involved  in  even  greater  doubt, 
being  regarded  by  some  investigators  as  contractile  elements  like 
those  of  the  spindle,  by  others  as  rigid  supporting  fibres  like  those 
of  the  central  spindle.  In  either  case  one  of  their  functions  is  prob- 
ably to  hold  the  kinetic  centre  in  a  fixed  position  while  the  chromo- 
somes are  pulled  apart.  Whether  they  *play  any  part  in  division  of 
the  cell-body  is  unknown ;  but  it  must  be  remembered  that  the  size 
of  the  aster  is  directly  related  to  that  of  the  resulting  cell  (p.  51)  — 
a  fact  which  indicates  a  very  intimate  relation  between  the  aster  and 
the  dividing  cell-body.  On  the  other  hand,  in  amitosis  the  cell-body 
may  divide  in  the  absence  of  asters. 

These  facts  show  that  mitosis  is  due  to  the  co-ordinate  play  of  an 
extremely  complex  system  of  forces  which  are  as  yet  scarcely  com- 
prehended. Its  purpose  is,  however,  as  obvious  as  its  physiological 
explanation  is  difficult.  //  is  the  end  of  mitosis  to  divide  every  part  of 
the  chromatin  of  the  mother-cell  equally  betweeii  the  daiigJiter-miclei. 
All  the  other  operations  are  tributary  to  this.  We  may  therefore 
regard  the  mitotic  figure  as  essentially  an  apparatus  for  the  distri- 
bution of  the  hereditary  substance,  and  in  this  sense  as  the  especial 
instrument  of  inheritance. 


LITERATURE.     II 

Auerbach,  L.  —  Organologische  Studien.     Breslau,  1874. 

Van  Beneden,  E.  —  Recherches  sur  la   maturation  de  Toeuf,  la  fdcondation  et  la 

division  cellulaire  :  Arch,  de  Biol.,  IV.     1883. 
Van  Beneden   &  Neyt.  —  Nouvelles   recherches   sur  la  fecondation  et  la  division 

mitosique  chez  TAscaride  mdgalocephale :  Bull.  Acad.  roy.  de  Belgiqtte,  1887. 

III.  14,  iNo.  8. 
Boveri,  Th.  —  Zellenstudien :  I.Jena.  Zeitschr.,XX\.  1887;  U.  Ibid.  XXII.  1888; 

III.  Ibid.  XXIV.  1890. 
Brauer,  A.  —  (Jber   die   Encystirung  von   Actinosphaerium   Eichhorni :    Zeitschr. 

Wiss.  Zo'dl,  LVIII.  2.     1894. 
Driiner,  L. —  Studien  uber  den   Mechanismus   der  Zelltheilung.    Jena.   Zeitschr. , 

XXIX.,  II.     1894. 
Erlanger,  R.  von.  —  Die  neuesten  Ansichten  iiber  die  Zelltheilung  und  ihre  Mechanik  : 

Zo'dl.  Centralb.,  III.  2.     1896. 
Flemming,  W.,'92.  —  Entvvicklung  und  Stand  der  Kenntnisse  iiber  Amitose  :  Merkel 

und  BonneVs  Ergebnisse,  II.     1 892 . 
Id.  —  Zelle.     (See  introductory    list.     Also  general  list.) 


SUMMARY  AND    CONCLUSION  8/ 

Fol,  H.  — (See  List  IV.) 

Heidenhain,  M.  —  Cytomechanische  Studien:  Arch.  f.  Entwickmech.,  I.  4.     1895. 

Hermann,  F.  —  Beitrag  zur  Lehre  von  der  Entstehung  der  karyokinetischen  Spindel : 

Arch.  Mik.  Anat.,  XXXVII.     1891. 
Hertwig,  R.^^ —  Uber  Centrosoma  und  Centralspindel :   Sitz.-Ber.  Ges.  Morph.  und 

P/iys.     MYinchen,  1895,  Heft  I. 
Mark,  E.  L.—  (See  List  IV.) 

Reinke,  F.  — Zellstudien :  I.  Arch.  Mik.  Anat.,  XLIII.  1894;  II.  Ibid.  XLIV.  1894. 
Strasburger,  E.  —  Karyokinetische  ^xoh\^vc\^'.  Jahrb.  f.  VViss.  Botan.  XXVIII.  1895. 
Waldeyer,  W.  —  Uber  Karyokinese  und  ihre  Beziehungen  zu  den  Befruchtungsvor- 

gangen  :  Arch.  Mik.  Anat.,  XXXII.  1888.     Q.J.M.S.,  XXX.  1889-90. 


CHAPTER    III 


THE   GERM-CELLS 

"  Not  all  the  progeny  of  the  primary  impregnated  germ-cells  are  required  for  the  forma- 
tion of  the  body  in  all  animals;  certain  of  the  derivative  germ-cells  may  remain  unchanged 
and  become  included  in  that  body  which  has  been  composed  of  their  metamorphosed  and 
diversely  combined  or  confluent  brethren;  so  included,  any  derivative  germ-cell  may  com- 
mence and  repeat  the  same  processes  of  growth  by  inhibition  and  of  propagation  by  spon- 
taneous fission  as  those  to  which  itself  owed  its  origin;  followed  by  metamorphoses  and 
combinations  of  the  germ-masses  so  produced,  which  concur  to  the  development  of  another 
individual."  Richard  Ovven.i 

"  Es  theilt  sich  demgemass  das  befruchtete  Ei  in  das  Zellenmaterial  des  Individuums  und 
in  die  Zellen  fiir  die  Erhaltung  der  Art."  M.  Nussbaum.- 

The  germ  from  which  every  living  form  arises  is  a  single  cell,  de- 
rived by  the  division  of  a  parent-cell  of  the  preceding  generation. 
In  the  unicellular  plants  and  animals  this  fact  appears  in  its  simplest 
form  as  the  fission  of  the  entire  parent-body  to  form  two  new  and 
separate  individuals  like  itself.  In  all  the  multicellular  types  the 
cells  of  the  body  sooner  or  later  become  differentiated  into  two  groups 
which  as  a  matter  of  practical  convenience  may  be  sharply  distin- 
guished from  one  another.  These  are,  to  use  Weismann's  terms  :  ( i ) 
the  somatic  cells,  which  are  differentiated  into  various  tissues  by  which 
the  functions  of  individual  life  are  performed  and  which  collectively 
form  the  "body,"  and  (2)  \h^ germ-cells,  which  are  of  minor  signifi- 
cance for  the  individual  life  and  are  destined  to  give  rise  to  new 
individuals  by  detachment  from  the  body.  It  must,  however,  be  borne 
in  mind  that  the  distinction  between  germ-cells  and  somatic  cells  is 
not  absolute,  as  some  naturalists  have  maintained,  but  only  relative. 
The  cells  of  both  groups  have  a  common  origin  in  the  parent  germ- 
cell;  both  arise  through  mitotic  cell-division  during  the  cleavage  of 
the  ovum  or  in  the  later  stages  of  development ;  both  have  essentially 
the  same  structure  and  both  may  have  the  same  power  of  develo])- 
ment,  for  there  are  many  cases  in  which  a  small  fragment  of  the  body 
consisting  of  only  a  few  somatic  cells,  perhaps  only  of  one,  may  give 

1  Parthenogenesis,  p.  3,  1849. 

2  Arch.  Mik.  Anat.  XVIIL  p.  112,  18S0. 

88 


THE    GERM-CELLS 


89 


rise  by  regeneration  to  a  complete  body.  The  distinction  between 
somatic  and  germ-cells  is  an  expression  of  the  physiological  division 
of  labour ;  and  while  it  is  no  doubt  the  most  fundamental  and  impor- 
tant differentiation  in  the  multicellular  body,  it  is  nevertheless  to  be 
regarded  as  differing  only  in  degree,  not  in  kind,  from  the  distinctions 
between  the  various  kinds  of  somatic  cells. 

In  the  lowest  multicellular  forms,  such  as  Volvox  (Fig.  41),  the 
differentiation  appears  in  a  very  clear  form.  Here  the  body  consists 
of  a  hollow  sphere  the  walls  of  which  consist  of  two  kinds  of  cells. 
The  very  numerous  smaller  cells  are  devoted  to  the  functions  of  nutri- 


Fig.  41. —  Volvox,  showing  the  small  ciliated  somatic  cells  and  eight  large  germ-cells  (drawn 
from  life  by  J.  H.  Emerton). 


tion  and  locomotion,  and  sooner  or  later  die.  Eight  or  more  larger 
cells  are  set  aside  as  germ-cells,  each  of  which  by  progressive  fission 
may  form  a  new  individual  like  the  parent.  In  this  case  the  germ- 
cells  are  simply  scattered  about  among  the  somatic  cells,  and  no 
special  sexual  organs  exist.  In  all  the  higher  types  the  germ-cells 
are  more  or  less  definitely  aggregated  in  groups,  supported  and 
nourished  by  somatic  cells  specially  set  apart  for  that  purpose  and 
forming  distinct  sexual  organs,  the  ovaries  and  spermaries  or  their 
equivalents.  Within  these  organs  the  germ-cells  are  carried,  pro- 
tected, and  nourished  ;  and  here  they  undergo  various  differentia- 
tions to  prepare  them  for  their  future  functions. 


90  THE    GERM-CELLS 

In  the  earlier  stages  of  embryological  development  the  progeni- 
tors of  the  germ-cells  are  exactly  alike  in  the  two  sexes  and  are  in- 
distinguishable from  the  surrounding  somatic  cells.  As  development 
proceeds,  they  are  first  differentiated  from  the  somatic  cells  and  then 
diverge  very  widely  in  the  two  sexes,  undergoing  remarkable  transfor- 
mations of  structure  to  fit  them  for  their  specific  functions.  The 
structural  difference  thus  brought  about  between  the  germ-cells  is, 
however,  only  the  result  of  physiological  division  of  labour.  The 
female  germ-cell,  or  ovum,  supplies  most  of  the  material  for  the  body 
of  the  embryo  and  stores  the  food  by  which  it  is  nourished.  It  is 
therefore  very  large,  contains  a  great  amount  of  cytoplasm  more  or 
less  laden  with  food-matter  {^yolk  or  deiUoplasni),  and  in  many  cases 
becomes  surrounded  by  membranes  or  other  envelopes  for  the  pro- 
tection of  the  developing  embryo.  On  the  whole,  therefore,  the  early 
life  of  the  ovum  is  devoted  to  the  accumulation  of  cytoplasm  and  the 
storage  of  potential  energy,  and  its  nutritive  processes  are  largely 
constructive  or  anabolic.  On  the  other  hand,  the  male  germ-cell  or 
spermatozoon  contributes  to  the  mass  of  the  embryo  only  a  very 
small  amount  of  substance,  comprising  as  a  rule  only  a  single  nucleus 
and  a  centrosome.  It  is  thus  relieved  from  the  drudgery  of  making 
and  storing  food  and  providing  protection  for  the  embryo,  and  is 
provided  with  only  sufficient  cytoplasm  to  form  a  locomotor  appara- 
tus, usually  in  the  form  of  one  or  more  cilia,  by  which  it  seeks  the 
ovum.  It  is  therefore  very  small,  performs  active  movements,  and 
its  metabolism  is  characterized  by  the  predominance  of  the  destruc- 
tive or  katabolic  processes  by  which  the  energy  necessary  for  these 
movements  is  set  free.^  When  finally  matured,  therefore,  the  ovum 
and  spermatozoon  have  no  external  resemblance  ;  and  while  Schwann 
recognized,  though  somewhat  doubtfully,  the  fact  that  the  ovum  is  a 
cell,  it  was  not  until  many  years  afterwards  that  the  spermatozoon 
was  proved  to  be  of  the  same  nature. 


A.     The  Ovum 

The  animal  Qg%  (Figs.  42,  43  A)  is  a  huge  spheroidal  cell,  sometimes 
naked,  but  more  commonly  surrounded  by  one  or  more  membranes 
which  may  be  perforated  by  a  minute  opening,  the  micropyle,  through 
which  the  spermatozoon  enters  (Fig.  45).  It  contains  an  enormous 
nucleus  known  as  the  germinal  vesicle,  within  which  is  a  very  conspic- 

^  The  metabolic  contrast  between  the  germ-cells  has  been  fully  discussed  in  a  most  sug- 
gestive manner  by  Geddes  and  Thompson  in  their  work  on  the  Evolution  of  Sex  ;  and  these 
authors  regard  this  contrast  as  but  a  particular  manifestation  of  a  metabolic  contrast  charac- 
teristic of  the  sexes  in  general. 


THE    OVUM 


91 


uous  nucleolus  known  to  the  earlier  observers  as  the  germinal  spot. 
In  many  eggs  the  latter  is  single,  but  in  other  forms  many  nucleoli 
are  present,  and  they  are  sometimes  of  more  than  one  kind,  as  in 
tissue-cell 9<'^  In  its  very  early  stages  the  ovum  contains  a  centro- 
some,  but  this  afterwards  disappears  from  view,  and  as  a  rule  cannot 
be  discovered  until  the  final  stages  of  maturation  (at  or  near  the  time 
of  fertilization).  It 
is  then  found  to  lie 
just  outside  the  ger- 
minal vesicle  on  the 
side  nearest  the  egg- 
periphery  where  the 
polar  bodies  are  sub- 
sequently formed 
After  extrusion  of  the 
polar  bodies  (p.  131) 
the  egg-centrosome 
as  a  rule  degenerates 
and  disappears.  The 
itgg  thus  loses  the 
power  of  division 
which  is  afterwards 
restored  during  fer- 
tilization through  the 
introduction  of  a  new 
centrosome  by  the 
spermatozoon.  In 
parthenogenetic  eggs, 
on  the  other  hand, 
the  egg-centrosome 
persists,  and  the  ^^^ 
accordingly  retains  the  power  of  division  without  fertilization.  The 
disappearance  of  the  egg-centrosome  would,  therefore,  seem  to  be 
in  some  manner  a  provision  to  necessitate  fertilization  and  thus  to 
guard  against  parthenogenesis. 

The  egg-cytoplasm  almost  always  contains  a  certain  amount  of 
nutritive  matter,  the  yolk  or  deutoplasm,  in  the  form  of  solid  spheres 
or  other  bodies  suspended  in  the  meshes  of  the  reticulum  and  vary- 
ing greatly  in  different  cases  in  respect  to  amount,  distribution,  form, 
and  chemical  composition. 

1  Hacker  ('95,  p.  249)  has  called  attention  to  the  fact  that  the  nucleolus  is  as  a  rule 
single  in  small  eggs  containing  relatively  little  deutoplasm  (coelenterates,  echinoderms, 
many  annelids,  and  some  copepods),  while  it  is  multiple  in  large  eggs  heavily  laden  with 
deutoplasm  (lower  vertebrates,  insects,  many  Crustacea). 


Fig.  42.  —  Ovarian  ^gg  of  the  sea-urchin  Toxopneustes 
(X750). 

g.v.  Nucleus  or  germinal  vesicle,  containing  an  irregular  dis- 
continuous network  of  chromatin ;  g.s.  nucleolus  or  germinal 
spot,  intensely  stained  with  hsematoxylin.  The  naked  cell-body 
consists  of  a  very  regular  network,  the  threads  of  which  appear 
as  irregular  rows  of  minute  granules  or  microsomes.  Below, 
at  s,  is  an  entire  spermatozoon  shown  at  the  same  enlargement 
(both  middle-piece  and  fiagellum  are  slightly  exaggerated  in 
size). 


92  THE    GERM-CELLS 

I.    The  Nucleus 

The  nucleus  or  germinal  vesicle  occupies  at  first  a  central  or  nearly 
central  position,  though  it  shows  in  some  cases  a  distinct  eccentricity 
even  in  its  earliest  stages.  As  the  growth  of  the  itgg  proceeds,  the 
eccentricity  often  becomes  more  marked,  and  the  nucleus  may  thus 
come  to  lie  very  near  the  periphery.  In  some  cases,  however,  the 
peripheral  movement  of  the  germinal  vesicle  occurs  only  a  very  short 
time  before  the  final  stages  of  maturation,  which  may  coincide  with 
the  time  of  fertilization.  Its  form  is  typically  that  of  a  spherical  sac, 
surrounded  by  a  very  distinct  membrane  (Fig.  42);  but  during  the 
growth  of  the  Q,gg  it  may  become  irregular  or  even  amoeboid  (Fig.  58), 
and,  as  Korschelt  has  shown  in  the  case  of  insect-eggs,  may  move 
through  the  cytoplasm  towards  the  source  of  food.  Its  structure  is 
on  the  whole  that  of  a  typical  cell-nucleus,  but  is  subject  to  very  great 
variation,  not  only  in  different  animals,  but  also  in  different  stages  of 
ovarian  growth.  Sometimes,  as  in  the  echinoderm  ovum,  the  chro- 
matin forms  a  beautiful  and  regular  reticulum  consisting  of  numer- 
ous chromatin-granules  suspended  in  a  network  of  linin  (Fig.  42). 
In  other  cases,  no  true  reticular  stage  exists,  the  nucleus  containing 
throughout  the  whole  period  of  its  growth  the  separate  daughter-chro- 
mosomes of  the  preceding  division  (copepods,  selachians,  amphibia),^ 
and  these  chromosomes  may  undergo  the  most  extraordinary  changes 
of  form,  bulk,  and  staining-reaction  during  the  growth  of  the  Q^g?'  It 
is  a  very  interesting  and  important  fact  that  during  the  growth  and 
maturation  of  the  ovum  a  large  part  of  the  chromatin  of  the  germinal 
vesicle  may  be  lost,  either  by  passing  out  bodily  into  the  cytoplasm, 
by  conversion  into  supernumerary  or  accessory  nucleoli  which  finally 
degenerate,  or  by  being  cast  out  and  degenerating  at  the  time  the 
polar  bodies  are  formed  (p.  177). 

The  nucleolus  of  the  egg-cell  is  here,  as  elsewhere,  a  variable 
quantity  and  is  still  imperfectly  understood.  The  nucleoli  are  of 
two  different  kinds,  either  or  both  of  which  may  be  present.  One 
of  these,  the  so-called  principal  nucleolus,  is  a  rounded,  usually  single 
body,  staining  intensely  with  the  same  dyes  that  colour  the  chromatin, 
and  often  containing  one  or  more  vacuoles.  This  is  typically  shown 
in  the  echinoderm  ^%g,  in  the  eggs  of  many  annelids,  mollusks,  and 
coelenterates,  in  some  Crustacea,  in  mammals,  and  in  some  other 
cases.  From  its  staining-reaction  this  type  of  nucleolus  appears 
to  correspond  in  a  chemical  sense  not  with  the  **true  nucleoli"  of 
tissue-cells,  but  with  the  net-knots  or  karyosomes,  such  as  the  nucle- 
oli of  nerve-cells  and  of  many  gland-cells  and  epithelial  cells.     The 

1  p.  193-  ^  P-  245- 


THE    OVUM  93 

second  form  comprises  the  so-called  "  accessory  nucleoli,"  which 
stain  less  intensely,  are  often  numerous,  and  perhaps  correspond 
with  the  true  nucleoli  of  tissue-cells.  As  growth  proceeds,  they 
usually  increase  in  size  and  number,  and  may  finally  become  very 
numerous,  in  which  case  they  often  occupy  a  peripheral  position 
in  the  germinal  vesicle.  This  is  typically  shown  in  amphibia  and 
selachians,  where  there  are  a  large  number  of  nucleoli,  which  are 
at  first  scattered  irregularly  through  the  germinal  vesicle  but  at  a 
certain  period  migrate  towards  the  periphery.  In  some  of  the 
mollusks  and  Crustacea  both  forms  coexist ;  but  even  closely  related 
species  may  differ  in  this  regard.  Thus,  in  Cyclops  brevicornis, 
according  to  Hacker,  the  very  young  ovum  contains  a  single  in- 
tensely chromatic  nucleolus ;  at  a  later  period  a  number  of  paler 
accessory  nucleoli  appear;  and  still  later  the  principal  nucleolus 
disappears,  leaving  only  the  accessory  ones.  In  C.  strcniuis,  on 
the  other  hand,  there  is  throughout  but  a  single  nucleolus.  In 
some  of  the  mollusks  and  annelids  the  *'  germinal  spot "  is  double, 
consisting  of  a  deeply  staining  principal  nucleolus  and  a  paler 
accessory    nucleolus    lying    beside    it,    as    in    Cyclas   and    in    Nereis 

(Fig.  43). 

The  physiological  meaning  of  the  nucleoli  is  still  involved  in 
doubt.  Many  cases  are,  however,  certainly  known  in  which  the 
nucleolus  plays  no  part  in  the  later  development  of  the  nucleus, 
being  cast  out  or  degenerating  in  situ  at  the  time  the  polar  bodies  are 
formed.  It  is,  for  example,  cast  out  bodily  in  the  medusa  y^qiiorea 
(Hacker)  and  in  various  annelids  and  echinoderms,  afterwards  lying 
for  some  time  as  a  "  metanucleus "  in  the  egg-cytoplasm  before 
degenerating.  In  many  cases  —  for  example  in  amphibia,  in  sela- 
chians, in  many  Crustacea,  annelids,  and  echinoderms  —  the  chromo- 
somes are  formed  in  the  germinal  vesicle  independently  of  the 
nucleoli  (Fig.  96),  which  degenerate  in  situ  when  the  membrane  of 
the  germinal  vesicle  disappears.  The  evidence  is,  therefore,  very 
strong  that  the  nucleoli  do  not  contribute  to  the  formation  of  the 
chromosomes,  and  that  their  substance  represents  passive  material 
which  is  of  no  further  direct  use.  There  is,  furthermore,  strong 
evidence  that  the  nucleoli  of  both  kinds  are  directly  or  indirectly 
derived  from  the  chromatin.  Hence  we  can  hardly  doubt  the 
conclusion  of  Hacker,  that  the  nucleoli  of  the  germ-cells  are  ac- 
cumulations of  by-products  of  the  nuclear  action,  derived  from  the 
chromatin  either  by  direct  transformation  of  its  substance,  or  as 
chemical    cleavage-products    or    secretions.^      It    will   be    shown    in 

"^  Hacker  regards  the  principal  nucleolus  as  a  more  highly  differentiated  modification  of 
the  accessory  nucleolus,  and  regards  it  as  a  pulsating  excretory  organ  comparable  with  the 
contractile  vacuoles  of  Protozoa. 


94  THE    GERM-CELLS 

Chapter  V.  that  in  some  cases  a  large  part  of  the  chromatic  reticulum 
is  cast  out,  and  degenerates  at  the  time  the  polar  bodies  are  formed. 
It  would  seem  that  the  nucleoli  may  likewise  represent  a  portion 
of  the  unused  chromatin,  more  closely  aggregated  and  more  or  less 
modified  in  a  chemical  sense. 

2.    The  Cytoplasm 

The  egg-cytoplasm  varies  greatly  in  appearance  with  the  varia- 
tions of  the  deutoplasm.  In  such  eggs  as  those  of  the  echino- 
derm  (Fig.  42),  which  have  little  or  no  deutoplasm,  the  cytoplasm 
forms  a  regular  reticulum,  which  is  perhaps  to  be  interpreted  as 
an  alveolar  structure.  Its  meshes  consist  of  closely  set  intensely 
staining  granules  or  microsomes  embedded  in  a  clearer  ground- 
substance.  The  latter,  which  fills  the  spaces  of  the  network,  is 
apparently  homogeneous,  and  contrasts  sharply  with  the  micro- 
somes in  staining  capacity.  In  eggs  containing  yolk  the  deutoplasm- 
spheres  or  granules  are  laid  down  between  the  meshes  of  the  net- 
work ;  and  if  they  are  very  abundant  the  latter  may  be  very  greatly 
reduced,  the  cytoplasm  assuming  a  pseudo-alveolar  structure  (Fig.  43), 
much  as  in  plant-cells  laden  with  reserve  starch.  In  many  cases 
a  peripheral  layer  of  the  ovum,  known  as  the  cortical  or  peri- 
vitelline  layer,  is  free  from  deutoplasm-spheres,  though  it  is  continu- 
ous with  the  protoplasmic  network  in  which  the  latter  lie  (Fig.  43). 
Upon  fertilization,  or  sometimes  before,  this  layer  may  disappear  by  a 
peripheral  movement  of  the  yolk,  as  appears  to  be  the  case  in  Nej-eis. 
In  other  cases  the  peri-vitelline  substance  rapidly  flows  towards  the 
point  at  which  the  spermatozoon  enters,  where  a  protoplasmic  germi- 
nal disc  is  then  formed  ;  for  example,  in  many  fish-eggs. 

The  character  of  the  yolk  varies  so  widely  that  it  can  here  be  con- 
sidered only  in  very  general  terms.  The  deutoplasm-bodies  are  com- 
monly spherical,  but  often  show  a  more  or  less  distinctly  rhomboidal 
or  crystalloid  form  as  in  amphibia  and  many  fishes,  and  in  such  cases 
they  may  sometimes  be  split  up  into  parallel  lamellae  known  as  yolk- 
plates.  Their  chemical  composition  varies  widely,  judging  by  the 
staining-reactions ;  but  we  have  very  little  definite  knowledge  on  this 
subject,  and  have  to  rely  mainly  on  the  results  of  analysis  of  the  total 
yolk,  which  in  the  hen's  Q,g^  is  thus  shown  to  consist  largely  of  pro- 
teids,  nucleo-albumins,  and  a  variety  of  related  substances  which  are 
often  associated  with  fatty  substances  and  small  quantities  of  car- 
bohydrates (glucose,  etc.).  In  some  cases  the  deutoplasm-spheres 
stain  intensely  with  nuclear  dyes,  such  as  haematoxylin ;  e.g.  in  many 
worms  and  mollusks  ;  in  other  cases  they  show  a  greater  affinity  for 
plasma-stains,  as  in  many  fishes    and    amphibia  and  in  the  annelid 


THE    OVUM 


95 


Fig.  43.  —  Eg^gs  of  the  annelid  Nereis,  before  and  after  fertilization,  X  400  (for  intermediate 
stages  see  Fig.  71). 

A.  Before  fertilization.  The  large  germinal  vesicle  occupies  a  nearly  central  position.  It  con- 
tains a  network  of  chromatin  in  which  are  seen  tive  small  darker  bodies;  these  are  the  quadruple 
chromosome-groups,  or  tetrads,  in  process  of  formation  (not  all  of  them  are  shown)  ;  these  alone 
persist  in  later  stages,  the  principal  mass  of  the  network  being  lost ;  g.s.  double  germinal  spot, 
consisting  of  a  chromatic  and  an  achromatic  sphere.  This  egg  is  heavily  laden  with  yolk,  in  the 
form  of  clear  deutoplasm-spheres  {d)  and  fat-drops  (/),  uniformly  distributed  through  the  cyto- 
plasm. The  peripheral  layer  of  cytoplasm  (peri-viielline  layer)  is  free  from  deutoplasm.  Outside 
this  the  membrane.  D.  The  ^-gg  some  time  after  fertilization  and  about  to  divide.  The  deuto- 
plasm is  now  concentrated  in  the  lower  hemisphere,  and  the  peri-vitelline  layer  has  disappeared. 
Above  are  the  two  polar  bodies  {p.b.).  Below  them  lies  the  mitotic  figure,  the  chromosomes 
dividing. 


96 


THE    GERM-CELLS 


en 


Nereis  (Fig.  43).      Often    associated  with    the   proper   deutoplasm- 
spheres  are  drops  of  oil,  either  scattered  through  the  yolk  (Fig.  43) 
or  united  to  form  a  single  large  drop,  as  in  many  pelagic  fish-eggs. 
The  deutoplasm  is  as  a  rule  heavier  than  the  protoplasm ;  and  in 

such  cases,  if  the  yolk  is  accumulated 
in  one  hemisphere,  the  ^^g  assumes  a 
constant  position  with  respect  to  gravity, 
the  egg-axis  standing  vertically  with  the 
animal  pole  turned  upward,  as  in  the 
frog,  the  bird,  and  many  other  cases. 
There  are,  however,  many  cases  in  which 
the  ^^g  may  lie  in  any  position.  When 
fat-drops  are  present  they  usually  lie  in 
the  vegetative  hemisphere,  and  since 
they  are  lighter  than  the  other  constitu- 
ents they  usually  cause  the  (tgg  to  lie 
with  the  animal  pole  turned  downwards, 
as  is  the  case  with  some  annelids 
{Nereis)  and  many  pelagic  fish-eggs. 


3.    The  Egg-envelopes 

The   egg-envelopes   fall  under  three 

categories.     These  are  :  — 
(a)   The  vitelline  membrane,  secreted 

by  the  ovum  itself. 
{U)   The  chorion,  formed   outside  the 
ovum    by    the     activity    of    the 
maternal  follicle-cells. 
(c)   Accessory    envelopes,    secreted    by 
the  walls  of  the  oviduct  or  other 
maternal     structures    after    the 
ovum  has  left  the  ovary. 
Only  the   first  of  these  properly  be- 
longs to  the  ovum,  the  second  and  third 
being  purely  maternal  products.     There 
are  some  eggs,  such  as  those  of  certain 
coelenterates    {e.g.     Renilld),    that    are 
naked  throughout  their  whole  develop- 
ment.      In  many  others,   of  which  the 
sea-urchin  is  a  type,  the  fresh-laid  Q^%'g 
is  naked    but   forms    a    vitelline    mem- 
brane  almost  instantaneously  after  the 


Fig.  44.  —  Schematic  figure  of  a 
median  longitudinal  section  of  the 
egg  of  a  fly  {Musca),  showing  axes 
ol  the  bilateral  eg'g,  and  the  mem- 
branes. [From  KORSCHELT  and 
Heider,  after  Henking  and  BlOCH- 

MANN.] 

e.n.  The  germ-nuclei  uniting;  vt., 
micropyle ;  p.b.  the  polar  bodies. 
The  flat  side  of  the  egg  is  the  dorsal, 
the  convex  side  the  ventral,  and  the 
micropyle  is  at  the  anterior  end. 
The  deutoplasm  (small  circles)  lies  in 
the  centre  surrounded  by  a  peripheral 
or  peri-vitelline  layer  of  protoplasm. 
The  outer  heavy  line  is  the  chorion, 
the  inner  lighter  line  the  vitelline 
membrane,  both  being  perforated  by 
the  micropyle,  from  whicli  exudes  a 
mass  of  jelly-like  substance. 


THE    OVUM  97 

spermatozoon  touches  it.^  In  other  forms  (insects,  birds)  the  vitelline 
membrane  may  be  present  before  fertilization,  and  in  such  cases  the 
*ig^,  is  often  surrounded  by  a  chorion  as  well.  The  latter  is  usually 
very  thick  and  firm  and  may  have  a  shell-like  consistency,  its  surface 
sometimes-^'showing  various  peculiar  markings,  prominences,  or  sculpt- 
ured patterns  characteristic  of  the  species  (insects).^ 

The  accessory  envelopes  are  too  varied  to  be  more  than  touched 
upon  here.  They  include  not  only  the  products  of  the  oviduct  or 
uterus,  such  as  the  albumin,  shell-membrane,  and  shell  of  birds  and 
reptiles,  the  gelatinous  mass  investing  amphibian  ova,  the  capsules 
of  molluscan  ova  and  the  like,  but  also  nutritive  fluids  and  capsules 
secreted  by  the  external  surface  of  the  body,  as  in  leeches  and  earth- 
worms. 

When  the  ^g^  is  surrounded  by  a  membrane  before  fertilization  it 
is  often  perforated  by  one  or  more  openings  known  as  micropyles, 
through    which    the    spermatozoa  ^ 

make  their  entrance  (Figs.  44, 
45).  Where  there  is  but  one  micro- 
pyle,  it  is  usually  situated  very 
near  the  upper  or  anterior  pole 
(fishes,  many  insects),  but  it  may 
be  at  the  opposite  pole  (some  in- 
sects and  mollusks),  or  even  on  Fig.  45.  —  Upper  pole  of  the  egg  of  ^7:^0- 
the  side  (insects).     In  many  insects    '"^''^''-    [Ussow.] 

thf-rp  U  p   o-rniin  nf  half  a   Hnypn  nr  '^^^  ^^^  '^   surrounded  by  a  very  thick 

there  is  a  group  01  nall  a  aozen  or  ,^embrane.  perforated  at  m  by  the  funnel- 
more    micropyleS    near    the     upper     shaped   micropyle;   below  the  latter  lies  the 

pole  of  the  egg,  and  perhaps  cor-  ;^r;  M^t''^o.^botr ^ '^'"  °' ''"'°" 
related  with  this  is  the  fact  that 

several  spermatozoa  enter  the  ^^g,  though  only  one  is  concerned 
with  the  actual  process  of  fertilization. 

The  plant  ovum,  which  is  usually  known  as  the  odsphere  (Figs. 
46,  80),  shows  the  same  general  features  as  that  of  animals,  being 
a  relatively  large,  quiescent,  rounded  cell  containing  a  large  nucleus. 
It  never,  however,  attains  the  dimensions  or  the  complexity  of  struct- 
ure shown  in  many  animal  eggs,  since  it  always  remains  attached  to 
the  maternal  structures,  by  which  it  is  provided  with  food  and  invested 
with  protective  envelopes.  It  is  therefore  naked,  as  a  rule,  and  is 
not  heavily  laden  with  reserve  food-matters  such  as  the  deutoplasm 
of  animal  ova.  A  vitelline  membrane  is,  however,  often  formed  soon 
after  fertilization,  as  in  echinoderms.     The  most  interesting  feature 

1  That  the  vitelline  membrane  does  not  pre-exist  seems  to  be  established  by  the  fact  that 
egg-fragments  likewise  surround  themselves  with  a  membrane  when  fertilized.     (Hertwig). 

^  In  some  cases,  according  to  Wheeler,  the  insect-egg  has  only  a  chorion,  the  vitelline 
membrane  being  absent. 
H 


98 


THE    GERM-CELLS 


of  the  plant-ovum  is  the  fact  that  it  often  contains  plastids  (leuco- 
plasts  or  chromatophores)  which,  by  their  division,  give  rise  to  those  of 

the  embryonic  cells.  These 
sometimes  have  the  form  of 
typical  chromatophores  con- 
taining pyrenoids,  as  in  Volvox 
and  many  other  algae  (Fig. 
46).  In  the  higher  forms 
(archegoniate  plants),  accord- 
ing to  the  researches  of 
Schmitz  and  Schimper,  the 
Q,^^  contains  numerous  mi- 
nute colourless  "  leucoplasts," 
which  afterwards  develop  into 
green  chromatophores  or  into 
the  starch-building  amylo- 
plasts.  This  is  a  point  of 
great  theoretical  interest ;  for 
the  researches  of  Schmitz, 
Schimper,  and  others  have 
rendered  it  highly  probable 
that  these  plastids  are  persistent  morphological  bodies  that  arise  only 
by  the  division  of  pre-existing  bodies  of  the  same  kind,  and  hence  may 
be  traced  continuously  from  one  generation  to  another  through  the 
germ-cells.  In  the  lower  plants  (algae)  they  may  occur  in  both  germ- 
cells  ;  in  the  higher  forms  they  are  found  in  the  female  alone  and  in 
such  cases  the  plastids  of  the  embryonic  body  are  of  purely  maternal 
origin. 


Fig.  46.  — Germ-cells  of  Volvox.  [Overton.] 
A.  Ovum  (oosphere)  containing  a  large  central 
nucleus  and  a  peripheral  layer  of  chromatophores; 
/.  pyrenoid.  B.  Spermatozoid  ;  c.v.  contractile  vacu- 
oles; e.  "eye-spot"  (chromoplastid)  ;  p.  pyrenoid. 
C.  Spermatozoid  stained  to  show  the  nucleus  (»). 


B.     The    Spermatozoon 

Although  spermatozoa  were  among  the  first  of  animal  cells  ob- 
served by  the  microscope,  their  real  nature  was  not  determined  for 
more  than  two  hundred  years  after  their  discovery.  Our  modern 
knowledge  of  the  subject  may  be  dated  from  the  year  1841,  when 
Kolliker  proved  that  they  were  not  parasitic  animalcules,  as  the 
early  observers  supposed,  but  the  products  of  cells  pre-existing  in  the 
parent  body.  Kolliker,  however,  did  not  identify  them  as  cells,  but 
believed  them  to  be  of  purely  nuclear  origin.  We  owe  to  Schweiggcr- 
Seidel  and  La  Valette  St.  George  the  proof,  simultaneously  brought 
forward  by  these  authors  in  1865,^  that  the  spermatozoon  is  a  com- 
plete cell,  consisting  of  nucleus  and  cytoplasm,  and  hence  of  the  same 


1  Arch.  Mik.  Anat.,  I.,  '65. 


THE   SPERMATOZOON 


99 


Apex  or  apical  body. 

Nucleus. 

End-knob  ( ?  centrosome). 
Middle-piece. 

Envelope  of  the  tail. 
.Axial  filarrent. 


morphological  nature  as  the  ovum.  It  is  of  extraordinary  minuteness, 
being  in  many  cases  less  than  ^'ooV'o "o  ^^^  ^^^^^  ^^  ^^^  ovum.^  Its 
precise  study  is  therefore  difficult,  and  it  is  not  surprising  that  our 
knowledge  of  its  structure  and  origin  is  still  far  from  complete. 

I.    Flagellate  Spermatozoa 

In  its  more   usual   form    the    animal    spermatozoon    resembles   a 
minute,  elongated  tadpole,  which  swims  very  actively  about  by  the 

vibrations  of  a  long,  slender  tail  morpho- 
logically comparable  with  a  single  cilium 
or  flagellum.  Such  a  spermatozoon  con- 
sists typically  of  four  parts,  as  shown  in 
Fig.  47:  — 

1.  The  nucleus,  which  forms  the  main 
portion  of  the  "  head,"  and  consists  of  a 
very  dense  and  usually  homogeneous  mass 
of  chromatin  staining  with  great  intensity 
with  the  so-called  "  nuclear  dyes "  {.e.g. 
haematoxylin  or  the  basic  anilines  such  as 
methyl-green).  It  is  surrounded  by  a  very 
thin  cytoplasmic  envelope. 

2.  A  minute  apex,  or  apical  body,  as  a 
rule  of  cytoplasmic  origin,  though  appar- 
ently derived  in  some  cases  from  the 
nucleus.  This  lies  at  the  front  end  of 
the  head,  and  in  some  cases  terminates 
in  a  sharp  spur  by  means  of  which  the 
spermatozoon  bores  its  way  into  the  ovum. 

3.  The  middle-piece,  or  connecting  piece, 
a  larger  cytoplasmic  body  lying  behind  the 
head  and  giving  attachment  to  the  tail. 
This  body  shows  the  same  staining-reaction 
as  the  tip,  having  an  especial  affinity  for 
"plasma-stains"  (acid  fuchsin,  etc.). 

4.  The  tail,  or  Jlagelliiin,  in  part,  at  least, 
a  cytoplasmic  product  developed  from  or 
in  connection  with  the  "  archoplasm  "  (at- 
traction-sphere   or  "Nebenkern")  of   the 

mother-cell.  It  consists  of  a  fibrillated  axial  filament  surrounded 
by  an  envelope  which  sometimes  shows  a  fibrillar  structure,  sometimes 
winds  spirally  about  the  axial  filament,  and  is  in  certain  cases  differ- 

1  In   the   sea-ur«hia,j /ij^t^/wzmj/t^,  I  jpstiuj^te   itii,  Kulk,  j^'Ss  being  between  yooWff  ^"d 
noVoo  t^^  volume  ofthfe  Oviim. '  The  incqualitj Is  in  man}^  oases  very  much  greater. 


End-piece. 


Fig 


47.  —  Diagram  of  the  flagel- 
late spermatozoon. 


[OO 


THE    GERM-CELLS 


entiated  into  a  fin-like  undulating  membrane.  The  axial  filament 
may  be  traced  through  the  middle-piece  up  to  the  head,  at  the  base  of 
which  it  terminates  in  a  minute  body,  single  or  double,  known  as 
the  end-knob,  and  not  improbably  representing  the  centrosome. 

There  is  still  some  doubt  regarding  the  nature  and  functions  of 
these  various  parts.  The  nucleus  is  proved  both  by  its  origin  and 
by  its  history  during  fertilization  to  be  exactly  equivalent  to  the 
nucleus  of  the  mature  ^^%.     The  middle-piece  and  the  tail  represent 


m— 


f 


Fig.  48.  — Spermatozoa  of  fishes  and  amphibia,     [Ballowitz.] 
A.  Sturgeon.    B.  Pike.    C.  D.  Leuciscus.     E.   Triton  (anterior  part).     E.    Triton  (posterior 
part  of  flagellum).     6^.  ^//ya  (anterior  part),    a.  apical  body ;   ^.end-piece;  /  flagellum  ;  >^.  end- 
knob  (?  centrosome)  ;  tn.  middle-piece;  «.  nucleus;  s.  apical  spur. 

the  principal  mass  of  the  cytoplasm  of  the  sperm-cell,  and  the  mid- 
dle-piece is  probably  to  be  regarded  as  merely  the  thickened  basal 
portion  of  the  flagellum. 

The  principal  uncertainty  relates  to  the  position  of  the  centro- 
some. It  is  certain  that  in  most  cases  the  centrosome  or  attraction- 
sphere  lies  in  the  middle-piece ;  for  from  it  the  centrosome  arises 
during  the  fertilizatior^  ;of  ;thc  egg,  in :  every /a(^curatej.y  known  case. 
In  a  few  cases,  moreover,' tile  iniddle-piece  has' been  traced  back  to 


THE   SPERMATOZOON  lOI 

the  attraction-sphere  of  the  mother-cell,  from  which  the  spermato- 
zoon is  formed  in  the  testis.  On  the  other  hand,  a  few  observers 
have  maintained,  apparently  on  good  evidence,  that  the  centrosome 
lies,  not  in  the  middle-piece,  but  at  the  apex  (p.   123). 

Reviewing  these  facts  from  a  physiological  point  of  view,  we  may 
arrange  the  parts  of  the  spermatozoon  under  two  categories  as 
follows :  — 

1.  The  essential  structures  which  play  a  direct  part  in  fertilization. 

These  are :  — 

{a)  The  nucleus^  which  contains  the  chromatin  and  is  to  be 
regarded  as  the  vehicle  of  inheritance. 

{U)  The  cent7vsoinej  certainly  contained  in  the  middle-piece  as 
a  rule,  though  perhaps  lying  in  the  tip  in  some  cases. 
This  is  the  fertilizing  element  par  excellence,  in  Boveri's 
sense,  since  when  introduced  into  the  ^gg  it  causes  the 
development  of  the  amphiaster  by  which  the  Qgg  divides. 

2.  The  accessory  structures,  which  play  no  direct  part  in  fertilization, 

viz. : — 

{a)  The  apex  or  spur,  by  which  the  spermatozoon  attaches  itself 
to  the  Qgg  or  bores  its  way  into  it. 

(^)  The  tail,  a  locomotor  organ  which  carries  the  nucleus  and 
centrosome,  and,  as  it  were,  deposits  them  in  the  Qgg  at 
the  time  of  fertilization.  There  can  be  little  doubt  that 
the  substance  of  the  flagellum  is  contractile,  and  that  its 
movements  are  of  the  same  nature  as  those  of  ordinary 
cilia.  Ballowitz's  discovery  of  its  fibrillated  structure  is 
therefore  of  great  interest,  as  indicating  its  structural  as 
well  as  physiological  similarity  to  a  muscle-fibre.  More- 
over, as  will  appear  beyond,  it  is  nearly  certain  that  the 
contractile  fibrillae  are  derived  from  the  attraction-sphere 
of  the  mother-cell,  and  therefore  arise  in  the  same  manner 
as  the  archoplasm-fibres  of  the  mitotic  figure  —  a  conclu- 
sion of  especial  interest  in  its  relation  to  Van  Beneden's 
theory  of  mitosis  (p.  70). 

Tailed  spermatozoa  conforming  more  or  less  nearly  to  the  type 
just  described  are  with  few  exceptions  found  throughout  the  Metazoa 
from  the  coelenterates  up  to  man ;  but  they  show  a  most  surprising 
diversity  in  form  and  structure  in  different  groups  of  animals,  and 
the  homologies  between  the  different  forms  have  not  yet  been  fully 
determined.  The  simpler  forms,  for  example  those  of  echinoderms 
and  some  of  the  fishes  (Figs.  48  and  74),  conform  very  nearly  to  the 
foregoing  description.      Every  part  of  the  spermatozoon  may,  how- 


102 


THE    GERM-CELLS 


H  \ 


Fig.  49,  —  Spermatozoa  of  various  animals.  [A-I,  L,  from  Ballowitz;  J,  A',  from  VON 
BRUNN.] 

A.  (At  the  left).  Beetle  (Cc/Wj),  partly  macerated  to  show  structure  of  flagellum  ;  it  con- 
sists of  a  supporting  fibre  {s.f.)  and  a  fin-like  envelope  (/.)  ;  n.  nucleus;  a,  a.  apical  body  divided 
into  two  parts  (the  posterior  of  these  is  perhaps  a  part  of  the  nucleus).  B.  Insect  {Calathus), 
with  barbed  head  and  fin-membrane.  C.  Bird  {Phyllopneuste).  D.  Bird  {Muscicapa),  showing 
spiral  structure;  nucleus  divided  into  two  parts  («i,  ;/"-^)  ;  no  distinct  middlr-piece.  E.  Bu'finch ; 
spiral  membrane  of  head.  F.  Gull  {Larus)  with  spiral  middle-piece  and  apical  knob.  G.  H.  Giant 
spermatozoon  and  ordinary  form  of  Tadorna.  /,  Ordinary  form  of  the  same  stained,  showing 
apex,  nucleus,  middle-piece  and  flagellum.  J.  "  Vermiform  spermatozoon  "  and,  K.  ordinary 
spermatozoon  of  the  snail  Paludina.  L.  Snake  {Coluber),  showing  apical  body  (a),  nucleus, 
greatly  elongated  middle-piece  (»«),  and  flagellum  (/). 


THE   SPERMATOZOON  IO3 

ever,  vary  more  or  less  widely  from  it  (Figs.  48-50).  The  head 
(nucleus)  may  be  spherical,  lance-shaped,  rod-shaped,  spirally  twisted, 
hook-shaped,  hood-shaped,  or  drawn  out  into  a  long  filament;  and 
it  is  often  .divided  into  an  anterior  and  a  posterior  piece  of  different 
staining  capacity,  as  is  the  case  with  many  birds  and  mammals. 
The  apex  sometimes  appears  to  be  wanting  —  e.g.  in  some  fishes 
(Fig.  48).  When  present,  it  is  sometimes  a  minute  rounded  knob, 
sometimes  a  sharp  stylet,  and  in  some  cases  terminates  in  a  sharp 
barbed  spur  by  which  the  spermatozoon  appears  to  penetrate  the 
ovum  {Tritori),  In  the  mammals  it  seems  to  be  represented  by  a 
cap-like  structure,  the  so-called  "head-cap,"  which  in  some  forms 
covers  the  anterior  end  of  the  nucleus.  It  is  sometimes  divided  into 
two  distinct  parts,  a  longer  posterior  piece  and  a  knob-like  anterior 
piece  (insects,  according  to  Ballowitz). 

The  middle-piece  or  connecting-piece  shows  a  like  diversity 
(Figs.  48-50).  In  many  cases  it  is  sharply  differentiated  from 
the  flagellum,  being  sometimes  nearly  spherical,  sometimes  flattened 
like  a  cap  against  the  nucleus,  and  sometimes  forming  a  short 
cylinder  of  the  same  diameter  as  the  nucleus,  and  hardly  distin- 
guishable from  the  latter  until  after  staining  (newt,  earthworm). 
In  other  cases  it  is  very  long  (reptiles,  some  mammals),  and  is 
scarcely  distinguishable  from  the  flagellum.  In  still  others  (birds, 
some  mammals)  it  passes  insensibly  into  the  flagellum,  and  no 
sharply  marked  limit  between  them  can  be  seen.  In  many  of  the 
mammals  the  long  connecting-piece  is  separated  from  the  head  by 
a  narrow  *'  neck  "  in  which  the  end-knobs  lie,  as  described  below. 

Internally,  the  middle-piece  consists  of  an  axial  filament  and  an 
envelope,  both  of  which  are  continuous  with  those  of  the  flagellum. 
In  some  cases  the  envelope  shows  a  distinctly  spiral  structure,  like 
that  of  the  tail-envelope ;  but  this  is  not  always  visible.  The  most 
interesting  part  of  the,  middle-piece  is  the  ''end-knob"  in  which  the 
axial  filament  terminates,  at  the  base  of  the  nucleus.  In  some  cases 
this  appears  to  be  single.  More  commonly  it  consists  of  two  minute 
bodies  lying  side  by  side  (Fig.  50,  B,  D).  This  body  is  the  only 
structure  in  the  middle-piece  having  the  appearance  of  a  centrosome ; 
and  Hermann  conjectures  that  this  is  probably  its  real  nature. 

The  flagellum  or  tail  is  merely  a  locomotor  organ  which  plays 
no  part  in  fertilization.  It  is,  however,  the  most  complex  part  of 
the  spermatozoon,  and  shows  a  very  great  diversity  in  structure. 
Its  most  characteristic  feature  is  the  axial  filament ,  which,  as  Bal- 
lowitz has  shown,  is  composed  of  a  large  number  of  parallel  fibrillae, 
like  a  muscle-fibre.  This  is  surrounded  by  a  cytoplasmic  envelope, 
which  sometimes  shows  a  striated  or  spiral  structure,  and  in  which, 
or  in  connection  with  which,  may  be  developed  secondary  or  acces- 


04 


THE    GERM-CELLS 


sory  filaments  and  other  structures.     At  the  tip  the  axial  filament 
may  lose  its  envelope  and  thus  give  rise  to  the  so-called  "end-piece  " 

(Retzius).  In  Triton,  for 
example  (Fig.  48,  F\  the 
envelope  of  the  axial  fila- 
ment ("  principal  filament  ") 
gives  attachment  to  a  re- 
markable fin-like  membrane, 
having  a  frilled  or  undulat- 
ing free  margin  along  which 
is  developed  a  **  marginal 
filament."  Towards  the  tip 
of  the  tail,  the  fin,  and 
finally  the  entire  envelope, 
disappears,  leaving  only  the 
axial  filament  to  form  the 
end-piece.  After  macera- 
tion the  envelope  shows  a 
conspicuous  cross-striation, 
which  perhaps  indicates  a 
spiral  structure  such  as  oc- 
curs in  the  mammals.  The 
marginal  filament,  on  the 
other  hand,  breaks  up  into 
\  I  I  numerous    parallel    fibrillae, 

\  I  '  while  the  axial  filament  re- 

mains unaltered  (Ballowitz). 
A  fin-membrane  has  also 
been  observed  in  some  in- 
sects and  fishes,  and  has 
been  asserted  to  occur  in 
mammals  (man  included). 
Later  observers  have,  how- 
ever, failed  to  find  the  fin  in 
mammals,  and  their  obser- 
vations indicate  that  the 
axial  filament  is  merely  sur- 
rounded by  an  envelope 
which  sometimes  shows 
traces  of  the  same  spiral 
arrangement  as  that  which 
is  so  conspicuous  in  the  connecting-piece.  In  the  skate  the  tail 
has  two  filaments,  both  composed  of  parallel  fibrilloe,  connected  by 
a  membrane  and   spirally  twisted   about    each    other ;    a    somewhat 


Fig.  50.  —  Spermatozoa  of  mammals.  {A-F  from 
Ballovviiz.) 

A.  Badger  (living).  B.  The  same  after  staining. 
C.  Bat  {Vesperugo).  D.  The  same,  fiagellum  and 
middle-piece  or  connecting-piece,  showing  end-knobs. 
E.  Head  of  the  spermatozoon  of  the  bat  ( Khinolophus) 
showing  details.  F.  Head  of  spermatozoon  of  the  pig. 
G,  Opossum  (after  staining).  //.  Double  spermatozoa 
from  the  vas  deferens  of  tlie  opossum.     /.  Rat. 

he.  head-cap  (apex)  ;  k.  end-knob  (?  centrosome) ; 
m.  middle-piece;  n.  nucleus  (in  B,  E,  /^consisting 
of  two  different  parts). 


THE   SPERMATOZOON 


105 


similar  structure  occurs  in  the  toad.  In  some  beetles  there  is  a 
fin-membrane  attached  to  a  stiff  axial  "supporting  fibre"  (Fig.  49, 
A).  The  membrane  itself  is  here  composed  of  four  parallel  fibres 
which  differ  entirely  from  the  supporting  fibre  in  staining  capacity 
and  in  the  fact  that  each  of  them  may  be  further  resolved  into  a 
large  number  of  more  elementary  fibrillae. 


Fig.  ^51.  —  Unusual  forms  of  spermatozoa. 
A.  B.  C.  Living  amoeboid  spermatozoa  of  the  crustacean  Polyphemus.     [Zacharias.J 
D.  E.  Spermatozoa  of  crab,  Dromia.     F.  Of  Ethnsa,  G.  of  Maja,  H.  of  Inachus.  •  [Grobben.] 
/.  Spermatozoon  of  lobster, //(?war«j.     [Herrick.] 
y.  Spermatozoon  of  crab,  Porcellana.     [GROBBEN,] 


Many  interesting  details  have  necessarily  been  passed  over  in  the  foregoing 
account.  One  of  these  is  the  occurrence,  in  some  birds,  amphibia  (frog)^  and 
mollusks,  of  two  kinds  of  spermatozoa  in  the  same  animal.  In  the  birds  and 
amphibia  the  spermatozoa  are  of  two  sizes,  but  of  the  same  form,  the  larger  being 
known  as  ''giant  spermatozoa"  (Fig.  49,  G^  EI).  In  the  gasteropod  Paludina  the 
two  kinds  dififer  entirely  in  structure,  the  smaller  form  being  of  the  usual  type  and 
not  unlike  those  of  birds,  while  the  larger,  or  "  vermiform,'''  spermatozoa  have  a 
worm-like  shape  and  bear  a  tuft  of  cilia  at  one  end,  somewhat  like  the  spermatozoids 
of  plants  (Fig.  49,  J.  K)  In  this  case  only  the  smaller  spermatozoa  are  functional 
(von    Brunn) . 

No  less  remarkable  is  the  conjugation  of  spermatozoa  in  pairs  (Fig.  50,  //),  whi«:h 


io6 


THE    GERM-CELLS 


takes  place  in  the  vas  deferens  in  the  opossum  (Selenka)  and  in  some  insects 
(Ballovvitz,  Auerbach).  Ballowitz's  researches  ('95)  on  the  double  spermatozoa 
of  beetles  {Dytiscidce)  prove  that  the  union  is  not  primary,  but  is  the  result  of  an 
actual  conjugation  of  previously  separate  spermatozoa.  Not  merely  two,  but  three 
or  more  spermatozoa  may  thus  unite  to  form  a  *•  spermatozeugma,"  which  swims  like 
a  single  spermatozoon.  Whether  the  spermatozoa  of  such  a  group  separate  before 
fertilization  is  unknown;  but  Ballowitz  has  found  the  groups,  after  copulation,  in 
the  female  receptaculum,  and  he  believes  that  they  may  enter  the  egg  in  this  form. 
The  physiological  meaning  of  the  process  is  unknown. 


2.    Other  Forms  of  Spermatozoa 


The  principal  deviations  from  the  flagellate  type  of  spermatozoon 
occur  among  the  arthropods  and  nematodes  (Fig.  51).  In  many  of 
these  forms  the  spermatozoa  have  no  flagellum,  and  in  some  cases  they 
are  actively  amoeboid;  for  example,  in  the  daphnid  PolyphemiLS  (Fig. 
51,  A^  B,  C)  as  described  by  Leydig  and  Zacharias.  More  commonly 
they  are  motionless  like  the  ovum.  In  the  chilognathous  myriapods 
the  spermatozoon  has  sometimes  the  form  of  a  bi-convex  lens  {Poly- 
desmiis),  sometimes  the  form  of  a  hat  or  helmet  having  a  double  brim 
{Jidits).  In  the  latter  case  the  nucleus  is  a  solid  disc  at  the  base  of 
In    many  decapod   Crustacea  the  spermatozoon   consists 

of  a  cylindrical  or  conical  body 
w  from  one  end  of  which  radiate 

a  number  of  stiff  spine-like 
processes.  The  nucleus  lies 
near  the  base.  In  none  of 
these  cases  has  the  centrosome 
been  identified. 


the    hat. 


3.    Paternal    Gerjn-cells    of 
Plants 

In  the  flowering  plants  the 
male  germ-cell  is  represented 
by  a  "generative  nucleus,"  to- 
gether with  two  centrosomes 
and  a  small  amount  of  cyto- 
plasm, lying  at  the  tip  of  the 
pollen-tube  (Fig.  80,  A).  On 
the  other  hand,  in  a  large  number  of  the  lower  plants  (Pteridophytes, 
Muscineae,  and  many  others),  the  male  germ-cell  is  a  minute  actively 
swimming  cell,  known  as  the  spermatozoid,  which  is  closely  analogous 


B 


Fig.  52.  —  Spermatozoids  of  Chara.  [Belajeff.] 
A.  Mother-cells  with  reticular  nuclei.    D.  Later 

stage,  with  spermatozoids  form injj.     C.  Mature  sper- 

matozoid  (the  elongate  nucleus  black). 


THE   SPERMATOZOON 


107 


to  the  spermatozoon.  The  spermatozoids  are  in  general  less  highly 
differentiated  than  spermatozoa,  and  often  show  a  distinct  resemblance 
to  the  asexual  swarmers  or  zoospores  so  common  in  the  lower  plants 
(Figs.  52,  ^-3).  They  differ  in  two  respects  from  animal  spermatozoa; 
first  in  possessing  not  one  but  two  or  several  flagella ;  second,  in  the 
fact  that  these  are 
attached  as  a  rule  not 
to  the  end  of  the  cell, 
but  on  the  side.  In 
the  lower  forms  plas- 
tids  are  present  in 
the  form  of  chromato- 
phores,  one  of  which 
may  be  differentiated 
into  a  red  *'  eye-spot,' 
as  in  Volvox  and 
FiiciLS  (Figs.  41,  53, 
A),  and  they  may 
even  contain  contrac- 
tile vacuoles  (Volvox) ; 
but  both  these,  struct- 
ures are  wanting  in 
the  higher  forms. 
These  consist  only  of 
a  nucleus  with  a  very 
small  amount  of  cyto- 
plasm, and  have  typi- 
cally a  spiral  form. 
In  Charay  where  their 
structure  and  devel- 
opment have  recently 
been  carefully  studied 
by  Belajeff,  the  sper- 
matozoids have  an 
elongated  spiral  form 
with  two  long  flagella 


Fig-  53. —Spermatozoids  of  plants.  [A,  B,  C,  E,  after 
GuiGNARD;  D,  F,  after  Strasburger.] 

A.  Of  an  alga  {Fucus)  ;  a  red  chromatophore  at  the  right 
of  the  nucleus.  B.  Liverwort  {Pellia).  C.  Moss  {Sphagnum). 
D.  Marsilia.  E.  Fern  {Angiopteris).  F.  Fern,  Phegopferis  (the 
nucleus  dark). 


attached  near  the 
pointed  end  which  is 
directed  forwards  in  swimming  (Fig.  52).  The  main  body  of  the 
spermatozoid  is  occupied  by  a  dense,  apparently  homogeneous  nu- 
cleus surrounded  by  a  very  delicate  layer  of  cytoplasm.  Behind  the 
nucleus  lies  a  granular  mass  of  cytoplasm,  forming  one  end  of  the 
cell,  while  in  front  is  a  slender  cytoplasmic  tip  to  which  the  flagella 
are  attached.     Nearly  similar  spermatozoids  occur  in  the  liverworts 


108  THE    GERM-CELLS 

and  mosses.  In  the  ferns  and  other  pteridophytes  a  somewhat  dif- 
ferent type  occurs  (Fig.  53).  Here  the  spermatozoid  is  twisted  into 
a  conical  spiral  and  bears  numerous  cilia  attached  along  the  upper 
turns  of  the  spire.  The  nucleus  occupies  the  lower  turns,  and 
attached  to  them  is  a  large  spheroidal  cytoplasmic  mass,  which  may, 
however,  be  cast  off  when  the  spermatozoid  is  set  free  or  at  the  time 
it  enters  the  archegonium.  This,  according  to  Strasburger,  proba- 
bly corresponds  to  the  basal  cytoplasmic  mass  of  Chara.  The  upper 
portion  of  the  spire  to  which  the  cilia  are  attached  is  composed  of 
cytoplasm  alone,  as  in  Chara, 

The  homologies,  or  rather  analogies,  between  the  respective  parts 
of  the  spermatozoid  and  spermatozoon  are  not  yet  very  definitely 
established,  since  the  history  of  the  spermatozoid  in  fertilization  has 
not  yet  been  accurately  followed.  Strasburger  ('92)  believes  that  the 
anterior  cytoplasmic  region,  to  which  the  cilia  are  attached,  consists 
of  "  kinoplasm  "  (archoplasm),  and  hence  corresponds  with  the  mid- 
dle-piece of  the  spermatozoon.  If  this  view  be  correct,  there  is,  on 
the  whole,  a  rather  close  correspondence  between  spermatozoid  and 
spermatozoon,  the  flagella  being  attached  in  both  cases  to  that  end  of 
the  cell  which  contains  the  centrosome  or  kinetic  centre,  the  nucleus 
lying  in  the  middle,  while  the  opposite  end  consists  of  cytoplasm  {i.e. 
the  apex  of  the  spermatozoon,  the  cytoplasmic  vesicle  of  pterido- 
phytes, the  basal  cytoplasm  of  Chara,  etc.).  The  attachment  of  the 
flagella  in  both  cases  to  the  archoplasmic  region  is  a  significant  fact, 
for  Strasburger  believes  that  they  arise  from  the  "  kinoplasm  "  (archo- 
plasm), and  it  is  probable  that  the  spermatozoon  tail  has  a  similar 
origin  (p.  126). 

C.     Origin  and  Growth  of  the  Germ-cells 

Both  ova  and  spermatozoa  take  their  origin  from  cells  known  as 
primordial  germ-cells,  which  become  clearly  distinguishable  from  the 
somatic  cells  at  an  early  period  of  development,  and  are  at  first  exactly 
alike  in  the  two  sexes.  What  determines  their  subsequent  sexual 
differentiation  is  unknown  save  in  a  few  special  cases.  From  such 
data  as  we  possess,  there  is  very  strong  reason  to  believe  that,  with 
a  few  exceptions,  the  primordial  germ-cells  are  sexually  indifferent, 
i.e.  neither  male  nor  female,  and  that  their  transformation  into  ova 
or  spermatozoa  is  not  due  to  an  inherent  predisposition,  but  is  a  reac- 
tion to  external  stimulus.  The  nature  of  the  stimulus  appears  to 
vary  in  different  cases.  Thus  Maupas's  experiments  seem  to  show 
conclusively  that,  in  rotifers,  the  differentiation  may  depend  on 
temperature,  a   high  temperature  tending  to  produce  males,   a  low 


ORIGIN  AND    GROWTH   OF   THE    GERM- CELLS 


09 


temperature,  females ;  while  those  of  Mrs.  Treat  on  lepidoptera  and 
of  Yung  on  amphibia  seem  to  leave  no  doubt  that  the  differentiation 
here  depends  on  the  character  of  the  nutrition,  highly-fed  individuals 
producing^ a  great  preponderance  of  females,  while  those  that  are 
underfed  give  rise  to  a  preponderance  of  males.  These  and  a  multi- 
tude of  related  observations  by  many  botanists  and  zoologists  render 
it  certain  that  sex  as  such  is  not  inherited.  What  is  inherited  is,  in 
Busing's  words,  only  the  particular  manner  in  which  one  or  the  other 
sex  comes  to  development.  The  dcterminatiojt  of  sex  is  not  by  in- 
heritance, but  by  the  combined  effect  of  external  conditions.^  In 
some  of  the  rotifers,  however,  sex  is  predetermined  from  the  begin- 


•  .^ '% 

^ 

-    %'     't 

m 

-  W  ^:^/* 

0 

e    ,  y   a 

Fig-  54-  —  Germ-cells  in  the  hydro-medusa,  Hydractinia.     [BUNTING.] 
A.  Section  through  young  medusa-bud,  with  very  young  ova  {ov.)    lying  in  the  entoderm; 
B.  Mature  gonophore,  showing  two  ova  lying  between  ectoderm  and  entoderm. 


ning,  the  eggs  being  of  two  sizes,  of  which  the  larger  produce  females ; 
the  smaller,  males. 

In  the  greater  number  of  cases,  the  primordial  germ-cells  arise  in 
a  germinal  epithelium  which,  in  the  coelenterates  (Fig.  54),  may  be  a 
part  of  either  the  ectoderm  or  entoderm,  and,  in  the  higher  types,  is  a 
modified  region  of  the  peritoneal  epithelium  lining  the  body-cavity. 
In  such  cases  the  primordial  germ-cells  may  be  scarcely  distinguish- 
able at  first  from  the  somatic  cells  of  the  epithelium.  But  in  other 
cases  the  germ-cells  may  be  traced  much  farther  back  in  the  develop- 
ment, and  they  or  their  progenitors  may  sometimes  be  identified  in 
the  gastrula  or  blastula  stage,  or  even  in  the  early  cleavage-stages. 
Thus  in  the  worm  Sagitta,  Hertwig  has  traced  the  germ-cells  back  to 

1  See  DU'sing,  '84;  Geddes,  Sex,  in  Encyclopedia  Britannica  ;  Geddes  and  Thompson, 
The  Evolution  of  Sex;  Watase,  On  the  Phenomena  of  Sex-differentiation,  '92. 


iO 


THE    GERM-CELLS 


two  primordial  germ-cells  lying  at  the  apex  of  the  archenteron.  In 
some  of  the  insects  they  appear  still  earlier  as  the  products  of  a  large 
"pole-cell"  lying  at  one  end  of  the  segmenting  ovum,  which  divides 


Fig-  55-  —  Origin  of  the  primordial  germ-cells  and  casting  out  of  chromatin  in  the  somatic 
cells  of  Ascaris.     [BOVERI.] 

A.  Two-cell  stage  dividing;  s.  stem-cell,  from  which  arise  the  germ-cells.  B.  The  same  from 
the  side,  later  in  the  second  cleavage,  showing  the  two  types  of  mitosis  and  the  casting  out  of 
chromatin  {c)  in  the  somatic  cell.  C.  Resulting  4-ceIl  -stage;  the  eliminated  chromatin  at  c. 
D.  The  third  cleavage,  repeating  the  foregoing  process  in  the  two  upper  cells. 


into  two  and  finally  gives  rise  to  two  symmetrical  groups  of  germ- 
cells.  Haecker  has  recently  traced  very  carefully  the  origin  of  the 
primordial  germ-cells  in  Cyclops  from  a  "stem-cell"  (Fig.  56)  clearly 
distinguishable  from  surrounding  cells  in  the  early  blastula  stage,  not 


I 


ORIGIN  AND    a  IW IV  III    OF   I 'HE    GERM-CELLS  III 

only  by  its  size,  but  also  by  its  large  nuclei  rich  in  chromatin,  and  by 
its  peculiar  mode  of  mitosis,  as  described  beyond. 

The  most  beautiful  and  remarkable  known  case  of  early  differenti- 
ation of  the  germ-cells  is  that  of  Ascaris,  where  Boveri  was  able  to 
trace  therA  back  continuously  through  all  the  cleavage-stages  to  the 
two-cell  stage!  Moreover,  from  the  outset  the  progenitor  of  the  germ- 
cells  differs  from  the  somatic  cells  not  only  in  tJie  greater  size  and  richness 
of  cJiromatin  of  its  nuclei,  biU  also  in  its  mode  of  mitosis ;  for  in  all 
those  blastomeres  destined  to  produce  somatic  cells  a  portion  of  the 
chromatin  is  cast  out  into  the  cytoplasm,  where  it  degenerates,  and 
only  in  the  germ-cells  is  the  sum  total  of  the  chromatin  retained.  In 
Ascaris  megalocephala  univalens  the  process  is  as  follows  (Fig.  55): 
Each  of  the  first  two  cells  receives  two  elongated  chromosomes.  As 
the  ovum  prepares  for  the  second  cleavage,  the  two  chromosomes 
reappear  in  each,  but  differ  in  their  behaviour  (Fig.  $$,  A,  B).  In  one 
of  them,  which  is  destined  to  produce  only  somatic  cells,  the  thickened 
ends  of  each  chromosome  are  cast  off  into  the  cytoplasm  and  degen- 
erate. Only  the  thinner  central  part  is  retained  and  distributed  to 
the  daughter-cells,  breaking  up  meanwhile  into  a  large  number  of 
segments  which  split  lengthwise  in  the  usual  manner.  In  the 
other  cell,  which  may  be  called  the  stem-cell  (Fig.  55,  s),  all  the 
chromatin  is  preserved  and  the  chromosomes  do  not  segment  into 
smaller  pieces.  The  results  are  plainly  apparent  in  the  4-cell  stage, 
the  two  somatic  nuclei,  which  contain  the  reduced  amount  of  chro- 
matin, being  small  and  pale,  while  those  of  the  two  stem-cells  are  far 
larger  and  richer  in  chromatin  (Fig.  55,  C).  At  the  ensuing  division 
(Fig.  55,  D)  the  numerous  minute  segments  reappear  in  the  two 
somatic  cells,  divide,  and  are  distributed  like  ordinary  chromosomes ; 
and  the  same  is  true  of  all  their  descendants  thenceforward.  The 
other  two  cells  (containing  the  large  nuclei)  exactly  repeat  the 
history  of  the  two-cell  stage,  the  two  long  chromosomes  reappearing 
in  each  of  them,  becoming  segmented  and  casting  off  their  ends 
in  one,  but  remaining  intact  in  the  other,  which  gives  rise  to  two 
cells  with  large  nuclei  as  before.  This  process  is  repeated  five 
times  (Boveri),  or  six  (Zur  Strassen),  after  which  the  chromatin- 
elimination  ceases,  and  the  two  stem-cells  or  primordial  germ-cells 
thenceforward  give  rise  only  to  other  germ-cells  and  the  entire 
chromatin  is  preserved.  Through  this  remarkable  process  it  comes 
to  pass  that  in  this  animal  07ily  the  gei'm-cells  receive  the  sum 
total  of  tJie  egg-chromatin  handed  doivn  from  the  parent.  All  of  the 
somatic  cells  cofitain  only  a  portion  of  the  original  gei'm-suhstance. 
'*  The  original  nuclear  constitution  of  the  fertilized  Qgg  is  transmitted, 
as  if  by  a  law  of  primogeniture,  only  to  one  daughter-cell,  and  by  this 
again    to   one,  and    so    on ;    while    in    the  other   daughter-cells,   the 


112  THE    GERM-CELLS 

chromatin  in  part  degenerates,  in  part  is  transformed,  so  that  all  of 
the  descendants  of  these  side-branches  receive  small  reduced  nuclei."  ^ 
It  would  be  difficult  to  overestimate  the  importance  of  this  dis- 
covery ;  for  although  it  stands  at  present  an  almost  isolated  case,  yet 
it  gives  us,  as  I  believe,  the  key  to  a  true  theory  of  differentiation 
development,^  and  may  in  the  end  prove  the  means  of  explaining 
many  phenomena  that  are  now  among  the  unsolved  riddles  of  the  cell. 


Fig.  56.  —  Primordial  germ-cells  in  Cyclops.     [HaCKER.] 
A.  Young  embryo,  showing  stem-cell    {st).     B.  The  stem-cell  has   divided  into  two,  giving 
rise  to  the  primordial  germ-cell  {g).     C.  Later  stage,  in  section;  the  primordial  germ-cell  has 
migrated  into  the  interior  and  divided  into  two;  two  groups  of  chromosomes  in  each. 


Hacker  ('95)  has  shown  that  the  nuclear  changes  in  the  stem- 
cells  and  primordial  eggs  of  Cyclops  show  some  analogy  to  those  of 
Ascaiis,  though  no  casting  out  of  chromatin  occurs.  The  nuclei  are 
very  large  and  rich  in  chromatin  as  compared  with  the  somatic  cells, 
and  the  number  of  chromosomes,  though  not  precisely  determined, 
is  less  than  in  the  somatic  cells  (Fig.  56).  Vom  Rath,  working 
in  the  same  direction,  has  found  that  in  the  salamander  also  the 
number  of  chromosomes  in  the  early  progenitors  of  the  germ-cells 


'  Boveri,  '91,  p.  437. 


Cf.  p.  321. 


GROIVTII  AND   DIFFERENTIATION   OF    THE    GERM-CELLS      II3 

is  one-half  that  characteristic  of  the  somatic  cells.^  In  both  these 
cases,  the  chromosomes  are  doubtless  bivalent,  representing  two 
chromosomes  joined  together.  In  Ascaris,  in  like  manner,  each  of 
the  two  chromosomes  of  the  stem-cell  or  primordial  germ-cells  is 
probably  plurivalent,  and  represents  a  combination  of  several  units 
of  a  lower  order  which  separate  during  the  segmentation  of  the 
thread  when  the  somatic  mitosis  occurs. 


D.     Growth  and  Differentiation  of  the  Germ-cells 

I.    Tlie  Ovum 

(a)  GroivtJi  and  Nutritio7i.  —  Aside  from  the  transformations  of  the 
nucleus,  which  are  considered  elsewhere,  the  story  of  the  ovarian 
history  of  the  Q%g  is  largely  a  record  of  the  changes  involved  in 
nutrition  and  the  storage  of  material.  As  the  primordial  germ-cells 
enlarge  to  form  the  mother-cells  of  the  eggs,  they  almost  invariably 
become  intimately  associated  with  neighbouring  cells  which  not  only 
support  and  protect  them,  but  also  serve  as  a  means  for  the  elabora- 
tion of  food  for  the  growing  egg-cell.  One  of  the  simplest  arrange- 
ments is  that  occurring  in  ccelenterates,  where  the  o-g^^  lies  loose 
either  in  one  of  the  general  layers  or  in  a  mass  of  germinal  tissue, 
and  may  crawl  actively  about  among  the  surrounding  cells  like  an 
Amoeba?  More  commonly,  a  definite  association  is  established  be- 
tween the  Qgg  and  the  surrounding  cells.  In  one  of  the  most  fre- 
quent arrangements  the  ovarian  cells  form  a  regular  layer  or  follicle 
about  the  ovum  (Figs.  58,  60),  and  there  is  very  strong  reason  to 
believe  that  the  follicle-cells  are  immediately  concerned  with  the  con- 
veyance of  nutriment  to  the  ovum.  A  number  of  observers  have 
maintained  that  the  follicle-cells  may  actually  migrate  into  the  interior 
of  the  ^gg,  and  this  seems  to  be  definitely  established  in  the  case  of 
the  tunicates.'^  Such  cases  are,  in  any  case,  extremely  rare ;  and, 
as  a  rule,  the  material  elaborated  by  the  nutritive  cells  is  passed 
into  the  ^gg  in  solution.  Very  curious  and  suggestive  conditions 
occur  among  the  annelids  and  insects.  In  the  annelids,  the  nutri- 
tive cells  often  do  not  form  a  follicle,  but  in  some  forms  each  ^gg  is 
accompanied  by  a  single  nurse-cell,  attached  to  its  side,  with  which 
it  floats  free  in  the  body-cavity.  In  Ophryotrocha,  where  it  has  been 
carefully  described  by  Korschelt,  the  nurse-cell  is  at  first  much  larger 

1  Cf.  p.  194,  Chapter  V. 

2  It  has  been  asserted  that  the  eggs  in  such  cases  feed  on  the  other  cells  by  ingulfing 
them  bodily,  Amoeba-fashion.     This  is  probably  an  error. 

^  See  Floderus,  '95. 


114 


THE    GERM-CELLS 


than  the  ^^^  itself,  and  contains  a  large,  irregular  nucleus,  rich  in 
chromatin  (Fig.  57).  The  egg-cell  rapidly  grows,  apparently  at  the 
expense  of  the  nurse-cell,  which  becojnes  reduced  to  a  mere  rudi- 
ment attached  to  one  side  of  the  ^gg  and  finally  disappears.  There 
can  hardly  be  a  doubt,  as  Korschelt  maintains,  that  the  nurse-cell  is 
in  some  manner  connected  with  the  elaboration  of  food  for  the  grow- 
ing egg-cell;  and  the  intensely  chromatic  character  of  the  nucleus 
is  well  worthy  of  note  in  this  connection. 

Somewhat  similar  nurse-cells  occur  in  the  insects,  where  they  have 
been  carefully  described  by  Korschelt.  The  eggs  here  lie  in  a  series 
in  the   ovarian    "egg-tubes"    alternating   with    nutritive    cells  vari- 


Pig.  57.  —  Egg  and  nurse-cell  in  the  annelid,  Ophryotrocha.     [KORSCHELT.] 
A.  Young  stage,  the  nurse-cell  («),  larger  than  the  egg  {o),    B.  Growth  of  the  ovum.     C.  Late 
stage,  the  nurse-cell  degenerating. 

ously  arranged  in  different  cases.  In  the  butterfly  Vanessa,  each 
^^%  is  surrounded  by  a  regular  follicular  layer  of  cells,  a  few  of 
which  at  one  end  are  differentiated  into  nurse-cells.  These  cells 
are  very  large  and  have  huge  amoeboid  nuclei,  rich  in  chromatin 
(Fig.  58,  ^).  In  the  ear-wig,  Forficida,  the  arrangement  is  still  more 
remarkable,  and  recalls  that  occurring  in  OphryotrocJia.  Here  each 
^g^  lies  in  the  egg-tube  just  below  a  very  large  nurse-cell,  which, 
when  fully  developed,  has  an  enormous  branching  nucleus  as  shown 
in  Fig.  115.  In  these  two  cases,  again,  the  nurse-cell  is  characterized 
by  the  extraordinary  development  of  its  nucleus — a  fact  which 
points  to  an  intimate  relation  between  the  nucleus  and  the  metabolic 
activity  of  the  cell.^ 

In  all  these  cases  it  is  doubtful  whether  the  nurse-cells  are  sister- 


1  See  p.  254. 


GROWTH  AND   DIFFERENTIATION   OF   THE    GERM-CELLS 


15 


cells  of  the  ^^g  which  have  sacrificed  their  own  development  for  the 
sake  of  their  companions,  or  whether  they  have  had  a  distinct  origin 
from  a  very  early  period.  That  the  former  alternative  is  possible  is 
shown  by  .the  fact  that  such  a  sacrifice  occurs  in  some  animals  after 
the  eggs  have  been  laid.  Thus  in  the  earthworm,  Lnmbriais  terres- 
tris,  several  eggs  are  laid,  but  only  one  develops  into  an  embryo,  and 
the  latter  devours  the  undeveloped  eggs.  A  similar  process  occurs 
in  the  marine  gasteropods,  where  the  eggs  thus  sacrificed  may 
undergo  certain  stages  of  development  before  their  dissolution.^ 


Fig.  58.  — Ovarian  eggs  of  insects.     [Korschelt.] 
A.  Egg  of  the  butterfly,  Vanessa,  surrounded  by  its  follicle;  above,  three  nurse-cells  (n.c.)  with 
branching  nuclei;   ^.v.  germinal  vesicle.     B.  Egg  of  water-beetle,  Z?>'/w«j,  living;  the  egg  (^.t/.) 
lies  between  two  groups  of  nutritive  cells ;  the  germinal  vesicle  sends  amoeboid  processes  into  the 
dark  mass  of  food-granules. 


(l?)  Diffe7'eiitiatio7i  of  the  Cytoplasm  and  Deposit  of  Deiitoplasni.  — 
In  the  very  young  ovum  the  cytoplasm  is  small  in  amount  and  free 
from  deutoplasm.  As  the  ^^^  enlarges,  the  cytoplasm  increases 
enormously,  a  process  which  involves  both  the  growth  of  the  pro- 
toplasm and  the  formation  of  passive  deutoplasm-bodies  suspended 
in  the  protoplasmic  network.  During  the  growth-period  a  peculiar 
body  known  as  the  yolk-nucleus  appears  in  the  cytoplasm  of  many 
ova,  and  this  is  probably  concerned  in  some  manner  with  the  growth 

1  See  McMurrich,  '86. 


ii6 


THE    GERM-CELLS 


of  the  cytoplasm  and  the  formation  of  the  yolk.     Both  its  origin  and 
its  physiological  role  are,  however,  still  involved  in  doubt. 

The  deutoplasm  first  appears,  while  the  eggs  are  still  very  small, 
in  the  form  of  granules  which  seem  to  have  at  first  no  constant  posi- 
tion with   reference   to   the  egg-nucleus,  even  in  the  same  species. 


Fig.  59.  —  Young  ovarian  eggs,  showing  yolk-nuclei  and  deposit  of  deutoplasm. 

A.  Myriapod  {Geophilus)  with  single  "  yolk-nucleus  "  (perhaps  an  attraction-sphere)  and  scat- 
tered deutoplasm.     [Balbiani.] 

B.  The  same,  with  several  yolk-nuclei,  and  attraction-sphere,  s.     [Balhiani.] 

C.  Fish  {Scorpcztia) ,  with  deutoplasm  forming  a  ring  about  the  nucleus,  and  an  irregular  mass 
of  "eliminated  chromatin  "(?  yoik-nucleus).     [Van  Bambeke.] 

D.  Ovarian  egg  of  young  duck  (3  months)  surrounded  by  a  follicle,  and  containing  a  "  yolk- 
nucleus,"  j.«.    [Mertens.] 


Thus  Jordan  ('93)  states  that  in  the  newt  (^Diemyctybis)  the  yolk  may 
be  first  formed  at  one  side  of  the  ^gg  and  afterwards  spread  to  other 
parts,  or  it  may  appear  in  more  or  less  irregular  separate  patches 
which  finally  form  an  irregular  ring  about  the  nucleus,  which  at  this 
period  has  an  approximately  central  position.  In  some  amphibia 
the  deutoplasm   appears   near  the   periphery  and  advances  inwards 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS      WJ 

towards  the  nucleus.  More  commonly  it  first  appears  in  a  zone 
surrounding  the  nucleus  (Fig.  59,  C,  D)  and  advances  thence  towards 
the  periphery  (trout,  Henneguy ;  cephalopods,  Ussow).  In  still  others 
{e.g.  in  rnyriapods,  Balbiani)  it  appears  in  irregular  patches  scat- 
tered quite  irregularly  through  the  ovum  (Fig.  59,  A).  In  Branchi- 
piis  the  yolk  is  laid  down  at  the  centre  of  the  ^^g,  while  the  nucleus 
lies  at  the  extreme  periphery  (Brauer).  These  variations  show  in 
general  no  definite  relation  to  the  ultimate  arrangement  —  a  fact 
which  proves  that  the  eccentricity  of  the  nucleus  and  the  polarity  of 
the  ^^^  cannot  be  explained  as  the  result  of  a  simple  mechanical  dis- 
placement of  the  germinal  vesicle  by  the  yolk,  as  some  authors  have 
maintained.  Neither  do  they  support  the  view  that  the  actual  polar- 
ity of  the  Qg'g  exists  from  the  beginning.  They  probably  arise  rather 
through  the  varying  physiological  conditions  under  which  the  egg- 
formation  takes  place ;  but  these  have  not  yet  been  sufficiently 
analyzed.^ 

The  primary  origin  of  the  deutoplasm-grains  is  a  question  that 
really  involves  the  whole  theory  of  cell-action  and  the  relation  of 
nucleus  and  cytoplasm  in  metabolism.  The  evidence  seems  per- 
fectly clear  that  in  many  cases  the  deutoplasm  arises  in  situ  in  the 
cytoplasm  like  the  zymogen-granules  in  gland-cells.  But  there  is 
now  a  great  body  of  evidence  that  seems  to  show  with  equal  clear- 
ness that  a  part  of  the  egg-cytoplasm  is  directly  or  indirectly  derived 
from  the  nucleus.  There  is  no  question  that  a  large  part  of  the  sub- 
stance of  the  germinal  vesicle  is  thrown  out  into  the  cytoplasm  at  the 
time  of  maturation,  as  shown  with  especial  clearness  in  the  eggs  of 
amphibia,  echinoderms,  and  some  worms  {e.g.  in  Nereis,  Fig.  71). 
A  large  number  of  observers  have  maintained  that  a  similar  giv- 
ing off  of  solid  nuclear  substance  occurs  during  the  earlier  stages  of 
growth ;  and  these  observations  are  so  numerous  and  some  of  them 
are  so  careful,  that  it  is  impossible  to  doubt  that  this  process  really 
takes  place.  The  portions  thus  cast  out  of  the  nucleus  have  been 
described  by  some  authors  as  actual  buds  from  the  nucleus  (Bloch- 
mann,  Scharff,  Balbiani,  etc.),  as  separate  chromatin-rods  (Van  Bam- 
beke,  Erlanger),  as  portions  of  the  chromatic  network  (Calkins),  or  as 
nucleoli  (Balbiani,  Will,  Ley  dig).  There  is  no  evidence  that  such 
eliminated  nuclear  materials  directly  give  rise  to  deutoplasm-granules. 
They  would  seem,  rather,  to  have  the  value  of  food-matters  or  forma- 
tive substances  which  are  afterwards  absorbed  and  elaborated  by  the 
cytoplasm,  the  deutoplasm  being  a  new  deposit  in  the  cytoplasmic 
substance.  It  is,  however,  a  matter  of  great  interest  that  formed 
nuclear  elements  should  be  given  off  into  the  cytoplasm,  in  view  of 
the  general  role  of  the  nucleus  as  discussed  in  Chapter  VII. 

1  Cf.  p.  288. 


Ii8 


THE    GERM-CELLS 


(c)  Yolk-imcleus, — The  term  **  yolk-nucleus  "  has  been  applied  to 
various  bodies  or  masses  that  appear  in  the  cytoplasm  of  the  growing 
ovarian  ^g^ ;  and  it  must  be  said  that  the  word  has  at  present  no 
well-defined  meaning.  We  may  distinguish  two  extreme  types  of 
*' yolk-nuclei "  which  are  connected  by  various  transitional  forms. 
At  one  extreme  is  the  yolk-nucleus  proper,  as  originally  described  by 
von  Wittich  ('45)  in  the  eggs  of  spiders  and  later  by  Balbiani  ('93)  in 


Fig.  60.  —  Young  ovarian  eggs  of  birds  and  mammals.  [Mertens.] 
A,  Egg  of  young  magpie  (8  days),  surrounded  by  the  follicle  and  containing  germinal  vesicle 
and  attraction-sphere.  B.  Primordial  o^g^  (oogonium)  of  new-born  cat,  dividing.  C.  Egg  of 
new-born  cat  containing  attraction-sphere  (j),  and  centrosome.  D.  Of  young  thrush  surrounded 
by  follicle  and  containing  besides  the  nucleus  an  attraction-sphere  and  centrosome  {$),  and  a 
yolk-nucleus  C^'. «.).  E.  Of  young  chick  containing  nucleus,  attraction-sphere  and  fatty  deuto- 
plasm-spheres  (black).  F.  Egg  of  new-born  child,  surrounded  by  follicle  and  containing  nucleus 
and  attraction-sphere. 


those  of  myriapods,  having  the  form  of  a  single  well-defined  sphe- 
roidal mass  which  appears  at  a  very  early  period  and  persists  through- 
out the  later  ovarian  history.  At  the  other  extreme  are  '*  diffused 
yolk-nuclei"  having  the  form  of  numerous  irregular  and  ill-defined 
masses  scattered  through  the  cytoplasm,  as  described  by  Stuhlmann 
('86)  in  the  eggs  of  insects  and  more  recently  by  Calkins  and  Foot  in 
earthworms.     An  intermediate  form  is  represented  in  the  amphi])ia 


GROWTH  AND   DIFFERENTIATION   OF   THE    GERM-CELLS      I  I9 

(Jordan,  '93)  and  myriapods  (Balbiani,  '93),  where  the  Qgg  contains  a 
number  of  fairly  well  defined  yolk-nuclei.  In  Linnbriciis  the  "  yolk- 
nucleus  "  first  appears  as  a  single  irregular  deeply  staining  body 
closely  applied  to  the  nucleus  and  afterwards  breaks  up  into  numer- 
ous smaller  bodies  (Calkins,  '95). 

The  most  diverse  accounts  have  been  given  of  the  structure  and 
origin  of  these  problematical  bodies.  This  is  in  part  owing  to  the 
fact,  recently  pointed  out  by  Mertens,  that  two  entirely  different 
structures  have  been  confounded  under  the  one  term.  One  of  these 
is  the  attraction-sphere  of  the  young  ^gg  with  its  centrosome.  Such 
a  "yolk-nucleus"  has  been  described  by  Balbiani  in  the  eggs  of  the 
myriapod  GeopJiihis  (Fig.  59,  B).  The  other  is  a  body,  variously 
described  as  arising  from  the  nucleus  or  in  the  cytoplasm,  which  is 
not  improbably  concerned  in  some  manner  with  the  constructive 
metabolism  involved  in  the  growth  of  the  egg-cytoplasm  and  perhaps 
indirectly  concerned  with  the  formation  of  deutoplasm.  It  seems 
clear  that  the  latter  form  alone  should  receive  the  name  of  yolk- 
nucleus,  if  indeed  the  term  is  worth  retaining. 

Mertens  ('93)  has  recently  described  the  ova  of  a  number  of  birds 
and  mammals  (including  man)  as  containing  a  very  distinct  attrac- 
tion-sphere containing  one  or  more  intensely  staining  centrosomes 
(Fig.  60).  This  has,  however,  nothing  to  do  with  the  true  yolk- 
nucleus  which  may  sometimes  be  seen  in  the  same  ^gg,  lying  beside 
the  attraction-sphere  (Fig.  60,  D).  The  latter  sooner  or  later  fades 
away  and  disappears.  The  yolk-nucleus,  on  the  other  hand,  may  long 
persist.  This  observation  probably  explains  the  strange  result  reached 
by  Balbiani  in  the  case  of  myriapods  {Geophilus),  where  the  ''yolk- 
nucleus  "  is  described  as  arising  by  a  budding  of  the  nucleus,  yet  is 
identified  with  an  attraction-sphere  !  The  "yolk-nucleus  "  of  Balbiani 
has  here  the  typical  appearance  of  an  attraction-sphere,  surrounded 
by  rays  and  containing  two  or  several  centrosomes  or  centrioles. 
Besides  this,  however,  the  Qgg  contains  several  other  bodies  which 
are  described  as  arising  by  budding  off  from  the  nucleus  and  per- 
haps represent  the  true  yolk-nuclei  (Fig.  59,  B). 

The  origin  of  the  yolk-nucleus  proper  appears  to  differ  in  different 
cases.  Jordan's  observations  on  the  newt  seem  to  leave  no  doubt 
that  the  bodies  described  as  yolk-nuclei  in  this  animal  arise  in  situ  in 
the  cytoplasm ;  and  a  similar  origin  of  the  yolk-nucleus  has  been 
described  by  a  number  of  earlier  observers.  On  the  other  hand,  a 
number  of  observers  have  asserted  its  origin  from  the  nucleus,  either 
by  a  process  of  nuclear  budding,  by  a  casting  out  of  the  nucleolus  of 
separate  chromatin-rods,  or  of  portions  of  the  chromatic  reticulum. 
That  such  a  casting-out  of  nuclear  substance  occurs  during  the  ova- 
rian history  of  some  eggs  appears  to  be  well  established ;  but  it  is 


120 


THE    GERM-CELLS 


uncertain  whether  the  bodies  thus  arising  have  the  same  physiologi- 
cal significance  as  the  **  yolk-nuclei  "  of  cytoplasmic  origin.  Calkins 
('95,  i),  working  in  my  laboratory,  has  brought  forward  strong  evi- 
dence that  the  ** yolk-nucleus"  of  Lmnbriciis  is  derived  from  a  sub- 
stance nearly  related  with  chromatin  (Fig.  61).     The   yolk-nucleus 


Fig.  61. — Young  ovarian  eggs  of  the  earthworm  {Lmnbricus),  showing  yolk-nucleus. 
(Calkins.] 

A.  Very  early  stage;  the  irregular  yolk-nucleus  {y.  n.)  closely  applied  to  the  germinal  vesicle 
and  staining  like  chromatin.  B.  Later  stage;  the  yolk-nuclfus  separating  from  the  germinal 
vesicle  and  changing  its  staining-power.  C.  Still  later  stage;  the  yolk-nucleus  broken  up  into 
rounded  bodies  staining  like  the  cytoplasm. 


here  first  appears  as  an  irregular  granular  body  lying  directly  on  the 
nuclear  wall,  which  in  some  cases  appears  to  be  interrupted,  as  if  yolk- 
nucleus  and  chromatin  were  directly  in  continuity.  Later  the  yolk- 
nucleus  separates  from  the  germinal  vesicle  and  lies  beside  it  in  the 
cytoplasm.  It  finally  breaks  up  into  a  considerable  number  of  sec- 
ondary yolk-nuclei  scattered  through  the  egg.  The  action  of  differ- 
ential stains  at  different  periods  indicates  that  the  substance  of  the 


GROWTH  AND   DIFFERENTIATION  OF   THE    GERM-CELIS      121 

yolk-nucleus  is  nearly  related  with  chromatin,  if  not  directly  derived 
ifrom  it.  When  treated  with  the  Biondi-Ehrlich  mixture  (basic  methyl 
green,  acid  red  fuchsin),  the  yolk-nucleus  at  first  stains  green  like 
the  chroipatin,  while  the  cytoplasm  is  red,  and  this  is  the  case  even 
after  the  yolk-nucleus  has  quite  separated  from  the  nuclear  mem- 
brane. Later,  however,  as  the  yolk-nucleus  breaks  up,  it  loses  its 
nuclear  staining  power,  and  stains  red  like  the  cytoplasm. 

This  conclusion  is,  however,  disputed  in  a  later  work  by  Foot  ('96), 
who  maintains  that  the  yolk-nucleus  in  Allolobophora  is  not  of  nuclear 
but  of  ''archoplasmic  "  origin,  though  no  relation  between  it  and  an 
attraction-sphere  is  established.^  She  adds  the  very  interesting  dis- 
covery that  the  "polar  rings"  (cf.  p.  150)  are  probably  to  be  identi- 
fied with  the  yolk-nucleus,  or  are  at  least  derived  from  a  similar 
substance. 

Calkins's  observations  taken  in  connection  with  those  of  Balbiani, 
Van  Bambeke,  and  other  earlier  workers  give,  however,  strong  evi- 
dence, as  I  believe,  that  the  "yolk-nucleus"  of  Liunbriciis  is  de- 
rived, if  not  from  the  nucleus,  at  any  rate  from  a  substance  nearly 
related  with  chromatin,  which  is  afterwards  converted  into  cyto- 
plasmic substance.  It  is  certain,  in  this  case,  that  the  appearance 
of  the  yolk-nucleus  is  coincident  with  a  rapid  growth  of  cytoplasm ; 
but  we  cannot  suppose  that  the  latter  grows  entirely  at  the  expense 
of  the  yolk-nucleus.  More  probably  the  yolk-nucleus  supplies  certain 
materials  necessary  to  constructive  metabolism,  and  it  is  not  impos- 
sible that  these  may  be  ferments.  We  may  perhaps  interpret  in 
the  same  manner  the  elimination  of  separate  nuclear  elements  {i.e. 
not  forming  a  definite  yolk-nucleus)  as  described  by  Van  Bambeke, 
Mertens,  v.  Erlanger,  and  many  earlier  writers. 

The  meaning  of  the  yolk-nuclei  of  purely  cytoplasmic  origin  is 
very  obscure,  and  we  have  at  present  really  no  ground  for  assigning 
to  them  any  particular  function.  It  can  only  be  said  that  their 
appearance  coincides  in  time  approximately  with  the  period  of  great- 
est constructive  activity  in  the  cytoplasm,  but  there  is  no  evidence  of 
their  direct  participation  in  the  yolk-formation,  and  we  do  not  know 
whether  they  are  active  constructive  physiological  centres,  or  merely 
stores  of  reserve  substances  or  degeneration-products. 

1  Miss  Foot's  use  of  the  term  "  archoplasm  "  largely  deprives  the  word  of  the  definite 
meaning  attached  to  it  by  Boveri.  To  identify  as  "archoplasm"  everything  stained  by 
Lyons  blue  is  indeed  a  broad  use  of  the  term. 


122  THE    GERM-CELLS 


2.    Formation  of  the  Spermatozoon 

Owing  to  the  extreme  minuteness  of  the  spermatozoon,  the 
changes  involved  in  the  differentiation  of  its  various  parts  have 
always  been,  and  in  some  respects  still  remain,  among  the  most 
vexed  of  cytological  questions.  The  earlier  observations  of  Kolliker, 
Schweigger-Seidel,  and  La  Valette  St.  George,  already  mentioned, 
established  the  fact  that  the  spermatozoon  is  a  cell ;  but  it  required 
a  long  series  of  subsequent  researches  by  many  observers,  foremost 
among  them  La  Valette  St.  George  himself,  to  make  known  the 
general  course  of  spermatogenesis.  This  is,  briefly,  as  follows : 
From  the  primordial  germ-cells  arise  cells  known  as  spamatogoiiia} 
which  at  a  certain  period  pause  in  their  divisions  and  undergo  a  con- 
siderable growth.  Each  spermatogonium  is  thus  converted  into  a 
spermatocyte,  which  by  two  rapidly  succeeding  divisions  gives  rise  to 
four  spermatozoa,  as  follows.^  The  primary  spermatocyte  first 
divides  to  form  two  daughter-cells  known  as  spermatocytes  of  the 
second  order  or  sperm  mother-cells.  Each  of  these  divides  again  — 
as  a  rule,  without  pausing,  and  without  the  reconstruction  of  the 
daughter-nuclei  —  to  form  two  spermatids  or  sperm-cells.  Each  of 
the  four  spermatids  is  then  directly  transformed  into  a  single  sperma- 
tozoon, its  nucleus  becoming  very  small  and  compact,  its  cytoplasm 
giving  rise  to  the  tail  and  to  certain  other  structures.  The  number 
of  chromosomes  entering  into  the  nucleus  of  each  spermatid  and 
spermatozoon  is  always  one-half  that  characteristic  of  the  tissue-cells, 
and  this  reduction  in  number  is  in  many  cases  effected  during  the 
two  divisions  of  the  primary  spermatocyte.  In  some  cases,  however 
{e.g.  in  the  salamander),  the  reduced  number  appears  during  the  divi- 
sion of  the  spermatogonia  and  may  even  appear  in  the  very  early 
germ-cells  (cf.  p.  194)-  The  reduction  of  the  chromosomes,  which  is  the 
most  interesting  and  significant  feature  of  the  process,  will  be  con- 
sidered in  the  following  chapter,  and  we  are  here  only  concerned  with 
the  transformation  of  the  spermatid  into  the  spermatozoon.  All 
observers  are  now  agreed  that  the  nucleus  of  the  spermatid  is  directly 
transformed  into  that  of  the  spermatozoon,  the  chromatin  becoming 
extremely  compact  and  losing,  as  a  rule,  all  trace  of  its  reticular 
structure.  It  is  generally  agreed,  further,  that  the  envelope  of  the 
tail-substance  is  derived  from  the  cytoplasm  of  the  spermatid. 
Beyond  this  point  opinion  is  still  far  from  unanimous,  though  it  is 
probable    that    the    other    structures  —  viz.    the    axial    filament,    the 

1  The  terminology,  now  almost  universally  adopted,  is  due  to  La  Valette  St.  George.  Cf. 
Fig.  90. 

-  See  Fig.  91. 


GROWTH  AND   DIFFERENTIATION  OF   THE    GERM-CELLS      1 23 

middle-piece,  and  the  point  —  are  likewise  of  cytoplasmic  origin;  and 
it  is  certain  that  the  middle-piece  is  in  some  cases  derived  from  the 
attraction-sphere  of  the  spermatid,  and  contains  the  centrosome. 

As  the  spermatid  develops  into  the  spermatozoon  it  assumes  an 
elongate  form,  the  nucleus  lying  at  one  end  while  the  cytoplasm  is 
drawn  out  to  form  the  flagellum  at  the  opposite  end.  The  origin  of 
the  axial  filament  is  still  in  doubt.  Many  authors  (for  example, 
Flemming  and  Niessing)  have  described  it  as  growing  out  from  the 
nucleus ;  but  more  recent  work  by  Hermann,  Moore,  and  others, 
shows  that  this  is  probably  an  error  and  that  the  axial  filament  is 
derived  from  the  substance  of  the  attraction-sphere. 

The  greatest  uncertainty  relates  to  the  origin  of  the  middle-piece 
and  the  apex.  By  one  set  of  authors  the  centrosome  is  believed  to 
pass  into  the  point  of  the  spermatozoon  (Platner,  Field,  Benda,  Pre- 
nant) ;  by  another  set,  into  the  middle-piece  (Hermann,  Wilcox,  Cal- 
kins). That  the  latter  is  a  correct  view  is  absolutely  demonstrated 
by  the  fact  that  during  fertilization  the  centrosome  in  every  accu- 
rately known  case  is  derived  from  the  middle-piece  (amphibia,  echino- 
derms,  tunicates,  earthworm,  insects,  mollusks,  etc.).  The  observations 
of  Platner  and  others  in  support  of  the  other  view  are,  however,  too 
detailed  to  be  rejected  on  this  ground  alone,  and  it  is  not  impossible 
that  the  position  of  the  centrosome  may  vary  in  different  forms.  The 
uncertainty  is  due  to  the  difficulty  of  tracing  out  the  fate  of  the  cen- 
trosome and  archoplasmic  structures  of  the  spermatid.  It  is  certain 
that  each  spermatid  receives  a  centrosome  or  attraction-sphere  from 
the  preceding  amphiaster.  But  besides  the  centrosome  (attraction- 
sphere)  the  spermatid  may  also  contain  a  second  "achromatic"  body 
known  as  the  paramicleiis  (Nebenkern)  or  niitosome^  which  has  un- 
doubtedly been  mistaken  for  the  attraction-sphere  in  some  cases  ^  and 
to  this  circumstance  the  existing  confusion  may  be  in  part  due.  The 
concurrent  results  of  ~La  Valette  St.  George,  Platner,  and  several 
others  have  shown  that  the  ''  Nebenkern  "  is  derived  from  the  re- 
mains of  the  spindle-fibres ;  but  the  most  divergent  accounts  of  its 
later  history  have  been  given  by  different  investigators.  According 
to  Platner's  studies  on  the  hwttQr^y  Pygcsra  ('89),  it  consists  of  a  larger 
posterior  and  a  smaller  anterior  body,  which  he  calls  respectively  the 
large  and  small  mitosoma  (Fig.  62,  C).  The  former  gives  rise  to  the 
investment  of  the  axial  filament  of  the  tail,  the  latter  to  the  middle- 
piece,  while  the  "  centrosome  "  lies  at  the  anterior  end  of  the  nucleus 
at  the  "  apex"  (Fig.  62,  D).  Field  ('95)  reaches  an  essentially  similar 
result  in  the  echinoderm  spermatozoon,  the  single  "  Nebenkern " 
forming  the    middle-piece,  while  the  ''centrosome"  lies   at  the   tip 

^  Compare  the  confusion  between  yolk-nucleus  and  attraction-sphere  in  the  ovum,  p.  1 19. 


124 


THE    GERM-CELLS 


(Fig.  62,  B).  Benda  describes  the  *'  Nebenkern  "  in  the  mammals  as 
consisting  of  two  parts,  one  of  which  passes  backward  and  takes 
part  in  the  formation  of  the  tail-envelope,  while  the  other  passes 
forward  to  form  the  apex  (head-cap  or  apical  knob)  and  represents 
the  attraction-sphere  (archoplasm).  A  somewhat  similar  account 
was  given  by  Platner  of  the  "  Nebenkern  "  of  pulmonates.  Accord- 
ing to  the  more  recent  work  of  Moore  on  elasmobranchs,  both 
middle-piece  and  apex  are  derived  from  the  attraction-sphere,  the 
centrosome  passing  into  the  former  (Fig.  62,  A). 

The  work  of  Platner  and  Field  appears   to  have  been  carefully 


Fig.  62.  —  Formation  of  the  spermatozoon  from  the  spermatid. 

A.  Late  stage  of  spermatid  of  the  shark  ScyUium.     [Moore.] 

B.  Spermatid  of  starfish  Chcet aster.     [FiRLD.] 

C.  Spermatid  of  butteifly  FygcBra.     D.  Young  spermatozoon  of  the  same.     [PLATNER.] 

a.  apical  body;  a.f.  axial  filament;  c.  "centrosome;"  e.  envelope  of  tail ;  w.  middle-piece 
("  small  mitosoma"  of  Platner)  ;  n.  nucleus  ;  p.  paranucleus  ("  Nebenkern,"  or  "  large  mitosoma" 
of  Platner). 


done,  yet  there  is  good  reason  to  believe  that  both  these  observers 
are  in  error,  since  their  results  are  contradicted  by  the  history  of  the 
spermatozoon  in  fertilization.  As  regards  the  insects,  Henking's 
observations  on  the  fertilization  of  the  butterfly  Picris  leave  little 
doubt  that  the  sperm-centrosome  is  here  derived  from  the  middle- 
piece;  and,  moreover,  in  the  grasshopper  Caloptemis,  Wilcox  ('95)  has 
traced  the  centrosome  of  the  spermatid  into  the  middle-piece.  In 
the  case  of  echinoderms,  Boveri,  Mathews,  and  myself,  confirmed  by 
.several  later  observers,  have  independently  traced  the  sperm-centro- 
some to  the  middle-piece  during  fertilization,  and  have  shown  that 


GROWTH  AND  DIFFERENTIATION  OF   THE    GERM-CELLS      12$ 


Fol  was  in  error  in  referring  it  to  the  tip.  Field's  conclusion  is  there- 
fore almost  certainly  erroneous,  and  he  has  probably  confounded  the 
centrosome  with  the  '*  Nebenkern  "  or  paranucleus. 

Diametrycally  opposed,  moreover,  to  the  results  of  Platner  and 
Field  are  those  of  Hermann  ('89)  and  Calkins  ('95,  2)  on  amphibia 
and  earthworms,  and 
both  these  observers 
have  devoted  especial 
attention  to  the  origin 
of  the  middle-piece. 
The  evidence  brought 
forward  by  the  last- 
named  author,  whose 
preparations  I  have 
critically  examined, 
seems  perfectly  con- 
clusive that  the  at- 
traction-sphere or 
centrosome  passes 
into  the  middle-piece. 
The  ''  Nebenkern," 
which  is  rarely  pres- 
ent, appears  in  this 
case  to  take  no  part 
in  the  formation  of 
the  flagellum,  but 
degenerates  without 
further  change.  In 
the  salamander  the 
origin  of  the  middle- 
piece  has  been  care- 
fully studied  by 
Flemming  and  Her- 
mann. The  latter 
('89)  has  traced  the 
middle-piece  back  to 
an  *'  accessory  body  " 
(Nebenkorper),  which  he  believes  to  be  not  a  "Nebenkern" 
(derived  from  the  spindle-fibres),  but  an  attraction-sphere  derived 
from  the  aster  of  the  preceding  division,  as  in  Limibriais.  This 
body  differs  greatly  from  an  ordinary  attraction-sphere,  consisting 
of  the  following  three  parts  lying  side  by  side  in  the  cytoplasm 
(Fig.  63).  These  are :  {a)  a  colourless  sphere,  {b)  a  minute  rounded 
body  which  stains  red  with  saffranin   like  the  nucleoli  or  plasmo- 


Fig.  63.  —  Formation  of  the  spermatozoon  from  the  sper- 
matid in  the  salamander.     [HERMANN.] 

A.  Young  spermatid  showing  the  nucleus  above,  and  below 
the  colorless  sphere,  the  ring,  and  the  chromatic  spliere. 
B.  Later  stage,  showing  the  chromatic  sphere  and  ring  at  the 
base  of  tlie  nucleus.  C,  D,  E,  F.  Later  stuges.  showing  the 
transformation  of  the  chromatic  sphere  into  the  middle-piece  /». 


126  THE    GERM-CELLS 

somes  of  the  spermatid-nucleus,  {c)  a  ring-shaped  structure  staining 
purple  with  gentian  violet,  like  the  chromatin.  The  colourless  sphere 
ultimately  vanishes,  the  red  rounded  body  gives  rise  to  the  middle- 
piece,  while  the  ring  gives  rise  to  the  envelope  (fin)  of  the  flagellum. 
The  apex  or  spur  is  developed  from  the  nuclear  membrane.^  Her- 
mann's results  on  the  mouse  agree  in  a  general  way  with  those 
on  the  salamander ;  but  the  apex  (head-cap)  is  here  derived  from 
the  cytoplasm.  A  "  Nebenkorper"  lies  in  the  cytoplasm,  consisting 
of  a  pale  sphere  and  a  smaller  deeply  staining  body.  From  the 
latter  arises  the  "  end-knob,"  which  Hermann  accordingly  homolo- 
gizes  with  the  middle-piece  of  the  salamander  spermatozoon,  and 
from  it  the  axial  filament  appears  to  grow  out  into  the  flagellum. 
The  colourless  sphere  disappears  as  in  the  salamander,  and  the 
envelope  of  the  axial  filament  is  derived  from  the  cytoplasm.  Moore 
('95)  describes  the  flagellum  of  elasmobranchs  as  growing  out  from 
the  attraction-sphere  (archoplasm)  of  the  spermatid  (Fig.  62,  A). 

Sujnmajy.  —  The  foregoing  account  shows  that  our  positive  know- 
ledge of  the  formation  of  the  spermatozoon  still  rests  upon  a  some- 
what slender  basis.  But  despite  the  discrepancies  in  existing 
accounts,  all  agree  that  the  spermatozoon  arises  by  a  direct  meta- 
morphosis of  the  spermatid,  receiving  from  it  a  nucleus  and  a 
small  amount  of  cytoplasm  containing  a  centrosome  or  attraction- 
sphere.  All  agree,  further,  that  the  middle-piece  is  of  archoplasmic 
origin,  being  derived,  according  to  some  authors,  from  a  true  attrac- 
tion-sphere (or  centrosome);  according  to  others,  from  a  "  Neben- 
kern"  formed  from  the  spindle-fibres.  The  former  account  of  its 
origin  is  certainly  true  in  some  cases.  The  latter  cannot  be  accepted 
without  reinvestigation,  since  it  stands  in  contradiction  to  what  is 
known  of  the  middle-piece  in  fertilization,  and  is  possibly  due  to 
a  confusion  between  attraction-sphere  and  *'  Nebenkern."  Similar 
doubts  exist  in  regard  to  the  origin  of  the  apex,  which  is  variously 
described  as  arising  from  the  nuclear  membrane,  from  the  general 
cytoplasm,  from  the  "  Nebenkern,"  and  from  the  centrosome. 

Most  late  observers  agree,  further,  that  the  flagellum  is  developed 
in  intimate  relation  with  the  archoplasmic  material  (attraction-sphere 
or  "  Nebenkern  ").  This  conclusion  tallies  with  that  of  Strasburger, 
who  regards  the  flagella  of  plant-spermatozoids  as  derived  from  the 
"  kinoplasm  "  (archoplasm),  and  it  is  of  especial  interest  in  view  of 
Van  Beneden's  hypothesis  of  the  contractility  of  the  archoplasm- 
fibrillae.  It  is,  however,  possible  that  the  axial  filament  may  be 
derived  from  the  nucleus,  in  which  case  it  would  have  an  origin 
comparable  with  that  of  the  spindle-fibres  in  many  forms  of  mitosis. 

^  Flemming  describcl  the  middle-piece  as  arising  inside  the  nucleus  ;  but  Hermann's 
observations  leave  no  doubt  that  this  was  an  error. 


STAINING-REACTIONS   OF   THE    GERM-NUCLEI  12/ 


E.     Staining-reactions  of  the  Germ-nuclei 

It  was  pointed  out  by  Ryder  in  1883  that  in  the  oyster  the  germ- 
nuclei  stain  differently  in  the  two  sexes ;  for  if  the  hermaphrodite 
gland  of  this  animal  be  treated  with  a  mixture  of  saffranin  and  methyl- 
green,  the  egg-nuclei  are  coloured  red,  the  sperm-nuclei  bluish-green. 
A  similar  difference  was  afterwards  observed  by  Auerbach  ('91)  in 
the  case  of  many  vertebrate  germ-cells,  where  the  egg-nucleus  was 
shown  to  have  a  special  affinity  for  various  red  and  yellow  dyes 
(eosin,  fuchsin,  aurantia,  carmin),  while  the  sperm-nuclei  were  espe- 
cially stained  with  blue  and  green  dyes  (methyl-green,  aniline  blue, 
haematoxylin).  He  was  thus  led  to  regard  the  chromatin  of  the  ^^^ 
as  especially  "erythrophilous,"  and  that  of  the  sperm  as  '' cyanophi- 
lous."  That  the  distinction  as  regards  colour  is  of  no  value  has  been 
shown  by  Zacharias,  Heidenhain,  and  others ;  for  staining  agents 
cannot  be  logically  classed  according  to  colour,  but  according  to  their 
chemical  composition ;  and  a  red  dye,  such  as  saffranin,  may  in  a 
given  cell  show  the  same  affinity  for  the  chromatin  as  a  green  or  blue 
dye  of  different  chemical  nature,  such  as  methyl-green  or  haema- 
toxylin.  Thus  Field  has  shown  that  the  sperm-nucleus  of  Asterias 
may  be  stained  green  (methyl-green),  blue  (haematoxylin,  gentian 
violet),  red  (saffranin),  or  yellow  (iodine),  and  it  is  here  a  manifest 
absurdity  to  speak  of  "  cyanophilous  "  chromatin  (cf.  p.  243).  It  is 
certainly  a  very  interesting  fact  that  a  difference  of  staining-reaction 
exists  between  the  two  sexes,  as  indicating  a  corresponding  difference 
of  chemical  composition  in  the  chromatin;  but  even  this  has  been 
shown  to  be  of  a  transitory  character,  for  the  staining-reactions  of  the 
germ-nuclei  vary  at  different  periods  and  are  exactly  alike  at  the  time 
of  their  union  in  fertilization.  Thus  Hermann  has  shown  that  when 
the  spermatids  and  immature  spermatozoa  of  the  salamander  are 
treated  with  saffranin  (red)  and  gentian  violet  (blue),^  the  chromatic 
network  is  stained  blue,  the  nucleoH  and  the  middle-piece  red ;  while 
in  the  mature  spermatozoon  the  reverse  effect  is  produced,  the  nuclei 
being  clear  red,  the  middle-piece  blue.  A  similar  change  of  staining- 
capacity  occurs  in  the  mammals.  The  great  changes  in  the  staining- 
capacity  of  the  egg-nucleus  at  different  periods  of  its  history  are 
described  at  pp.  245,  246.  Again,  Watase  has  observed  in  the  newt 
that  the  germ-nuclei,  which  stain  differently  throughout  the  whole 
period  of  their  maturation,  and  even  during  the  earlier  phases  of 
fertilization,  become  more  and  more  alike  in  the  later  phases  and 
at  the  time  of  their  union  show  identical  staining-reactions.^     A  very 

'  By  Flemmjng's  triple  method.  '^  '92,  p.  492. 


128  THE    GERM-CELLS 

similar  series  of  facts  has  been  observed  in  the  germ-nuclei  of  plants 
by  Strasburger  (p.  163).  These  and  many  other  facts  of  like  import 
demonstrate  that  the  chemical  differences  between  the  germ-nuclei 
are  not  of  a  fundamental  but  only  of  a  secondary  character.  They 
are  doubtless  connected  with  the  very  different  character  of  the  meta- 
bolic processes  that  occur  in  the  history  of  the  two  germ-cells ;  and 
the  difference  of  the  staining-reaction  is  probably  due  to  the  fact 
that  the  sperm-chromatin  consists  of  pure  or  nearly  pure  nucleic  acid, 
while  the  egg-chromatin  is  a  nuclein  containing  a  much  higher  per- 
centage of  albumin. 

LITERATURE.     Ill 

Ballowitz,  E.  —  Untersuchungen  Uber  die  Struktur  der  Spermatozoen :  I.  {birds) 
Arch.  Mik.  Anat.  XXXII.,  1888;  2.  {insects)  Zeitschr.  Wiss.  Zool.^  L.,  1890; 
3.  {fishes^  amphibia,  reptiles)  Arch.  Mik.  Anat.,  XXXVI.,  1890;  4.  {mam- 
mals) Zeit.  Wiss.  Zool.,  LII.,  1891. 

Van  Beneden,  E.  —  Recherches  sur  la  composition  et  la  signification  de  Poeuf :  Mem. 
cour.  de  VAcad.  roy.  de  s.  de  Belgiqiie.,  1870. 

Boveri,  Th.  —  Uber  Differenzierung  der  Zellkerne  wahrend  der  Furchung  des  Eies 
von  Ascaris  meg. :  Anat.  Anz.,  1887. 

Brunn,  M.  von.  —  Beitrage  zur  Kenntniss  der  Samenkorper  und  ihrer  Entvvickelung 
bei  Vogeln  und  Saugethieren :  Arch.  Mik.  Anat.,  XXXIII.,  1889. 

Hacker,  V.  —  Die  Eibildung  bei  Cyclops  und  Camptocanthus :  Zool.  Jahrb.,  V., 
1892.     (See  also  List  V.) 

Hermann,  F.  —  Urogenitalsystem  ;  Struktur  und  Histiogenese  der  Spermatozoen  : 
Merkel  iind  Dojinefs  Ergebnisse,  II.,  1892. 

Kblliker,  A.  —  Beitrage  zur  Kenntniss  der  Geschlechtsverhaltnisse  und  der  Samen- 
flussigkeit  wirbelloser  Tiere.     Berlin,  1841. 

Leydig,  Fr.  —  Beitrage  zur  Kenntniss  des  thierischen  Eies  im  unbefruchteten  Zu- 
stande;  Zool.  Jahrb.,  III.      1889. 

Schweigger-Seidel,  F.  —  tjber  die  Samenkbrperchen  und  ihre  Entwicklung :  Arch. 
Mik.  Anat.,  I.     1865. 

Strasburger,  E.  —  Histologische  Beitrage ;  Heft  IV :  Das  Verhalten  des  Pollens 
und  die  Befruchtungsvorgange  bei  den  Gymnospermen,  Schwarmsporen,  pflanz- 
liche  Spermatozoiden  und  das  Wesen  der  Befruchtung.     Fischer,  Jena,  1892. 

Thomson,  Allen.  —  Article,  "Ovum,"  in  Todd's  Cyclopedia  of  Anatomy  and  Physi- 
ology.    1859. 

Waldeyer,  W.  —  Eierstock  und  Ei.     Leipzig,  1870. 

Id.  —  Bau  und  Entvvickelung  der  Samenfaden  :    Verh.  d.  Anat.  Ges.     Leipzig,  1887. 


CHAPTER    IV 

FERTILIZATION    OF   THE   OVUM 

"  It  is  conceivable,  and  indeed  probable,  that  every  part  of  the  adult  contains  molecules 
derived  both  from  the  male  and  from  the  female  parent;  and  that,  regarded  as  a  mass  of 
molecules,  the  entire  organism  may  be  compared  to  a  web  of  which  the  warp  is  derived 
from  the  female  and  the  woof  from  the  male."  Huxley.^ 

In  mitototic  cell-division  we  have  become  acquainted  with  the 
means  by  which,  in  all  higher  forms  at  least,  not  only  the  continuity 
of  life,  but  also  the  maintenance  of  the  species,  is  effected;  for  through 
this  beautiful  mechanism  the  cell  hands  on  to  its  descendants  an 
exact  duplicate  of  the  idioplasm  by  which  its  own  organization  is 
determined.  As  far  as  we  can  see  from  an  a  priori  point  of  view  there 
is  no  reason  why,  barring  accident,  cell-division  should  not  follow 
cell-division  in  endless  succession  in  the  stream  of  life.  It  is  possible, 
indeed  probable,  that  such  may  be  the  fact  in  some  of  the  lower  and 
simpler  forms  of  life  where  no  form  of  sexual  reproduction  is  known 
to  occur.  In  the  vast  majority  of  living  forms,  however,  the  series 
of  cell-divisions  tends  to  run  in  cycles  in  each  of  which  the  energy 
of  division  gradually  comes  to  an  end  and  is  on\y  restored  by  an 
admixture  of  living  matter  derived  from  another  cell.  This  operation, 
known  3.?>  fertilization  ox  fecundation,  is  the  essence  of  sexual  repro- 
duction ;  and  in  it  we  behold  a  process  by  which  on  the  one  hand 
the  energy  of  division  is  restored,  and  by  which  on  the  other  hand 
two  independent  lines  of  descent  are  blended  into  one.  Why  this 
dual  process  should  take  place  we  are  as  yet  unable  to  say,  nor 
do  we  know  which  of  its  two  elements  is  to  be  regarded  as  the 
primary  and  essential  one.  According  to  the  older  and  more  familiar 
"  dynamic  "  hypothesis,  brought  forward  by  Biitschli  ('76)  and  Minot 
^77 y  '79)  ^i^d  afterwards  supported  by  such  investigators  as  Engel- 
mann,  Hensen,  Hertwig,  and  Maupas,  the  essential  end  of  sexuality 
is  rejiivenescence,  i.e.  the  restoration  of  the  growth-energy  and  the 
inauguration  of  a  new  cycle  of  cell-division.  Maupas's  celebrated 
experiments  on  the  conjugation  of  Infusoria,  although  not  yet  ade- 

^  Evolution,  in  Science  and  Culture,  p.  296,  from  Enc.  Brit.,  1878. 

K  129 


I30  FERTILIZATION   OF   THE    OVUM 

quately  confirmed,  have  yielded  very  strong  evidence  that  in  these 
unicellular  animals,  even  under  normal  conditions,  the  processes  of 
growth  and  division  sooner  or  later  come  to  an  end,  undergoing  a 
process  of  natural  "senescence,"  which  can  only  be  counteracted  by 
conjugation.  That  conjugation  or  fertilization  actually  has  such  a 
dynamic  effect  is  disputed  by  no  one.  What  is  not  determined  is 
whether  this  is  the  primary  motive  for  the  process  —  i.e.  whether 
the  need  of  fertilization  is  a  primary  attribute  of  living  matter  —  or 
whether  it  has  been  secondarily  acquired  in  order  to  ensure  a  mixture 
of  germ-plasms  derived  from  different  sources.  The  latter  view  has 
been  urged  with  great  force  by  Weismann,  who  rejects  the  rejuve- 
nescence theory  i7i  toto  and  considers  the  essential  end  of  fertilization 
to  be  a  mixture  of  germ-plasms  (''Amphimixis")  as  a  means  for  the 
production,  or  rather  multiplication,  of  variations  which  form  the 
material  on  which  selection  operates.  On  the  other  hand,  Hatschek 
il^y,  i)  sees  in  fertilization  exactly  the  converse  function  of  checking 
variations  and  holding  the  species  true  to  the  specific  type.  The 
present  state  of  knowledge  does  not,  I  believe,  allow  of  a  decision 
between  these  diverse  views,  and  the  admission  must  be  made  that 
the  essential  nature  of  sexual  reproduction  must  remain  undetermined 
until  the  subject  shall  have  been  far  more  thoroughly  investigated, 
especially  in  the  unicellular  forms,  where  the  key  to  the  ultimate 
problem  is  undoubtedly  to  be  sought. 


A.    General  Sketch 

Among  the  unicellular  plants  and  animals,  fertilization  is  effected 
by  means  of  conjitgation^  3.  process  in  which  two  or  more  individuals 
permanently  fuse  together,  or  in  which  two  unite  temporarily  and 
effect  an  exchange  of  nuclear  matter,  after  which  they  separate.  In 
all  the  higher  forms  fertilization  consists  in  the  permanent  fusion  of 
two  germ-cells,  ofie  of  paternal  and  one  of  maternal  origin.  We  may 
first  consider  the  fertilization  of  the  animal  ^gg,  which  appears  to  take 
place  in  essentially  the  same  manner  throughout  the  animal  kingdom, 
and  to  be  closely  paralleled  by  the  corresponding  process  in  plants. 

Leeuwenhoek,  whose  pupil  Hamm  discovered  the  spermatozoa 
(1677),  put  forth  the  conjecture  that  the  spermatozoon  must  pene- 
trate into  the  o^gg ;  but  the  process  was  not  actually  seen  until  nearly 
two  centuries  later  (1854),  when  Newport  observed  it  in  the  case  of 
the  frog's  ^g% ;  and  it  was  described  by  Pringsheim  a  year  later  in  one 
of  the  lower  plants,  CEdigonium.  The  first  adequate  description  of 
the    process  was  given   by   Hermann   Fol,   in    1879,^  though   many 

^  See  V Ilenogenie,  pp.  124  ff.,  for  a  full  historical  account. 


GENERAL   SKETCH 


I'M 


earlier  observers,  from  the  time  of  Martin  Barry  ('43)  onwards,  had 
seen  the  spermatozoa  inside  the  egg-envelopes,  or  asserted  its  entrance 
into  the  Q,gg. 

In  many  cases  the  entire  spermatozoon  enters  the  ^gg  (mollusks, 
insects,  nematodes,  some  annelids,  Petroinyzon,  axolotl,  etc.),  and  in 
such  cases  the  long  flagellum  may  sometimes  be  seen  coiled  within 
the  ^gg  (Fig.  64).  Only  the  nucleus  and  middle-piece,  however,  are 
concerned  in  the  actual  fertilization ;  and  there  are  some  cases 
(echinoderms)  in  which  the  tail  is  left  outside  the  ^gg.     At  or  near 


Fig.  64.  —  Fertilization  of  the  &gg  of  the  snail  Physa.  [Kostanecki  and  Wierzejski.] 
A.  The  entire  spermatozoon  lies  in  the  ^g'g,  its  nucleus  at  the  right,  flagellum  at  the  left,  while 
the  minute  sperm-amphiaster  occupies  the  position  of  the  middle-piece.  The  first  polar  body  has 
been  formed,  the  second  is  forming.  B.  The  enlarged  sperm-nucleus  and  sperm-amphiaster  lie 
near  the  centre;  second  polar  body  forming  and  the  first  dividing.  The  egg-centrosomes  and 
asters  afterwards  disappear,  their  place  being  taken  by  those  of  the  spermatozoon. 


the  time  of  fertilization,  the  ^gg  successively  segments  off  at  the  upper 
pole  two  minute  cells,  known  as  the  polar  bodies  (Figs.  64,  65,  89)  or 
directive  corpuscles,  which  degenerate  and  take  no  part  in  the  subse- 
quent development.  This  phenomenon  takes  place,  as  a  rule,  imme- 
diately after  entrance  of  the  spermatozoon.  It  may,  however,  occur 
before  the  spermatozoon  enters,  and  it  forms  no  part  of  the  process 
of  fertilization  proper.  It  is  merely  the  final  act  in  the  process  of 
maturation,  by  which  the  ^gg  is  prepared  for  fertilization,  and  we 
may  defer  its  consideration  to  the  following  chapter. 


132  FERTIUZATJON   OF   THE    OVUM 


The  Genn-niic/i'i  in  FcrtiIir;ation 


The  modern  era  in  the  study  of  fertilization  may  be  said  to  begin 
with  Oscar  Hertwig's  discovery,  in  1875,  of  the  fate  of  the  sperma- 
tozoon within  the  ^^^.  Earlier  observers  had,  it  is  true,  paved  the 
way  by  showing  that,  at  the  time  of  fertilization,  the  Q.gg  contains 
two  nuclei  that  fuse  together  or  become  closely  associated  before 
development  begins.  (Warneck,  Biitschli,  Auerbach,  Van  Beneden, 
Strasburger.)  Hertwig  discovered,  in  the  Q:^g  of  the  sea-urchin 
{Toxopncustes  lividus),  that  one  of  tJiese  nuclei  belongs  to  the  egg, 
while  the  other  is  derived  from  the  spermatozoon.  This  result  was 
speedily  confirmed  in  a  number  of  other  animals,  and  has  since  been 
extended  to  every  species  that  has  been  carefully  investigated.  The 
researches  of  Strasburger,  De  Bary,  Schmitz,  Guignard,  and  others 
have  shown  that  the  same  is  true  of  plants.  In  every  known  case  an 
essential  phenomenon  of  fertilization  is  the  imion  of  a  sperm-nucleus, 
of  paternal  origin^  zvith  an  egg-nucleus,  of  maternal  origin,  to  form  the 
primary  nucle?ts  of  the  embryo.  This  nucleus,  knoivn  as  the  cleavage- 
or  segmentatiofi-nucleus,  gives  rise  by  division  to  all  the  nuclei  of  the 
body,  and  hence  every  nucleus  of  the  child  may  contain  nuclear  substance 
derived  from  both  parents.  And  thus  Hertwig  was  led  to  the  conclu- 
sion ('84),  independently  reached  at  the  same  time  by  Strasburger, 
Kolliker,  and  Weismann,  that  the  nucleus  is  the  most  essential  ele- 
ment concerned  in  hereditary  transmission. 

This  conclusion  received  a  strong  support  in  the  year  1883,  through 
the  splendid  discoveries  of  Van  Beneden  on  the  fertilization  of  the 
thread-worm,  Ascaris  megalocephala,  the  egg  of  which  has  since  ranked 
with  that  of  the  echinoderm  as  a  classical  object  for  the  study  of  cell- 
problems.  Van  Beneden's  researches  especially  elucidated  the  struct- 
ure and  transformations  of  the  germ-nuclei,  and  carried  the  analysis 
of  fertilization  far  beyond  that  of  Hertwig.  In  Ascaris,  as  in  all 
other  animals,  the  sperm-nucleus  is  extremely  minute,  so  that  at  first 
sight  a  marked  inequality  between  the  two  sexes  appears  to  exist  in 
this  respect.  Van  Beneden  showed  not  only  that  the  inequality  in 
size  totally  disappears  during  fertilization,  but  that  the  two  nuclei 
undergo  a  parallel  series  of  structural  changes  which  demonstrate 
their  precise  morphological  equivalence  down  to  the  minutest  detail ; 
and  here,  again,  later  researches,  foremost  among  them  those  of 
Boveri,  Strasburger,  and  Guignard,  have  shown  that,  essentially,  the 
same  is  true  of  the  germ-cells  of  other  animals  and  of  plants.  The 
facts  in  Ascaris  (variety  bivalens)  are  essentially  as  follows  (Fig. 
65)  :  After  the  entrance  of  the  spermatozoon,  and  during  the  for- 
mation of  the  polar  bodies,  the  sperm-nucleus  rapidly  enlarges  and 


GENERAL   SKETCH 
-pb 


133 


E  F 

Fig.  65.  —  Fertilization  of  the  egg  of  Ascaris  megalocephala,  var.  bivalens.  [BOVERI.]  (For 
later  stages  see  Fig.  104.) 

A.  The  spermatozoon  has  entered  the  egg,  its  nucleus  is  shown  at  cT;  beside  it  lies  the  granu- 
lar mass  of  "archoplasm"  (attraction-sphere);  above  are  tiie  closing  phases  in  the  formation  of 
the  second  polar  body  (two  chromosomes  in  each  nucleus) 0  B.  Germ-nuclei  (9,  cf)  in  the  reticu- 
lar stage ;  the  attraction-sphere  {a)  contains  the  dividing  centrosome.  C.  Chromosomes  forming 
in  the  germ-nuclei ;  the  centrosome  divided.  D.  Each  germ-nucleus  resolved  into  two  chromo- 
somes ;  attraction-sphere  {a)  double.  E.  Mitotic  figure  forming  for  the  first  cleavage ;  the  chro- 
mosomes {c)  already  split.  /\  First  cleavage  in  progress,  showing  divergence  of  the  daughter- 
chromosomes  towards  the  spindle-poles  (only  three  chromosomes  sliown). 


134 


FERTILIZATION  OF   THE    OVUM 


finally  forms  a  typical .  nucleus  exactly  similar  to  the  egg-nucleus. 
The  chromatin  in  each  nucleus  now  resolves  itself  into  two  long, 
worm-like  chromosomes,  which  are  exactly  similar  in  form,  size,  and 
staining  reaction  in  the  two  nuclei.  Next,  the  nuclear  membrane 
fades  away,  and  the  four  chromosomes  lie  naked  in  the  egg-substance.^ 
Every  trace  of  sexual  difference  has  now  disappeared,  and  it  is 
impossible  to  distinguish  the  paternal  from  the  maternal  chromo- 
somes (Figs.  65,  /),  E).  Meanwhile  an  amphiaster  has  been  devel- 
oped which,  with  the  four  chromosomes,  forms  the  mitotic  figure  for 
the  first  cleavage  of  the  ovum,  tJie  chromatic  portion  of  which  has 
been  synthetically  formed  by  the  tmion  of  tzvo  equal  germ-nticlei.    The 


A  B 

Fig.  66.  —  Germ-nuclei  and  chromosomes  in  the  eggs  of  nematodes.     [Carnoy.] 
A.  Egg  of  nematode  parasitic  in  Scylliuvi ;  the  two  germ-nuclei  in  apposition,  each  containing 
four  chromosomes  ;  the  two  polar  bodies  above.     B.  'Egg  oi  Fi/aroides ;  each  germ-nucleus  with 
eight  chromosomes ;  polar  bodies  above,  deutoplasm-spheres  below. 


later  phases  follow  the  usual  course  of  mitosis.  Each  chromosome 
splits  lengthwise  into  equal  halves,  the  daughter-chromosomes  are 
transported  to  the  spindle-poles,  and  here  they  give  rise,  in  the  usual 
manner,  to  the  nuclei  of  the  two-celled  stage.  Each  of  these  tmclei, 
therefore,  receives  exactly  equal  amounts  of  paternal  and  maternal 
chromatin. 

These  discoveries  were  confirmed  and  extended  in  the  case  of 
Ascaris  by  Boveri  and  by  Van  Beneden  himself  in  1887  and  1888 
and  in  several  other  nematodes  by  Carnoy  in  1887.  Carnoy  found 
the  number  of  chromosomes  derived  from  each  sex  to  be  in  Coronilla 
4,  in   Ophiosto7num  6,  and    in  Filaroides  8.       A    little    later   Boveri 


GENERAL   SKETCH  135 

('90)  showed  that  the  law  of  numerical  equality  of  the  paternal  and 
maternal  chromosomes  held  good  for  other  groups  of  animals,  being 
in  the  sea-urchin  Echinus  9,  in  the  worm  Sagitta  9,  in  the  medusa 
Tiara  14,  .and  in  the  vsxoVvd'i)^  PtcrotracJiea  16  from  each  sex.  Similar 
results  were  obtained  in  other  animals  and  in  plants,  as  first  shown  by 
tiuignard  in  the  lily  ('91),  where  each  sex  contributes  12  chromosomes. 
In  the  onion  the  number  is  8  (Strasburger) ;  in  the  annelid  Ophryo- 
trocJia  it  is  only  2  from  each  sex  (Korschelt).  In  all  these  cases  the 
number  contributed  by  each  is  one-half  the  number  characteristic  of  the 
body-cells.  The  union  of  two  germ-cells  thus  restores .  the  normal 
number,  and  thus  we  find  the  explanation  of  the  remarkable  fact 
commented  on  at  p.  48  that  tJie  number  of  chromosomes  in  sexually 
produced  organisms  is  alzvays  even} 

These  remarkable  facts  demonstrate  the  two  germ-nuclei  to  be  in 
a  morphological  sense  precisely  equivalent,  and  they  not  only  lend 
very  strong  support  to  Hertwig's  identification  of  the  nucleus  as  the 
bearer  of  hereditary  qualities,  but  indicate  further  that  these  qualities 
must  be  carried  by  the  chromosomes ;  for  their  precise  equivalence  in 
number,  shape,  and  size  is  the  physical  correlative  of  the  fact  that 
the  two  sexes  play,  on  the  whole,  equal  parts  in  hereditary  transmis- 
sion. And  thus  we  are  finally  led  to  the  view  that  chromatin  is  the 
physical  basis  of  inheritance,  and  that  the  smallest  visible  units 
of  structure  by  which  inheritance  is  effected  are  to  be  sought  in  the 
chromatin-granules  or  chromomeres. 


2.    The  Centrosojne  in  Fertilization 

The  origin  of  the  centrosomes  and  of  the  amphiaster,  by  means  of 
which  the  paternal  and  maternal  chromosomes  are  distributed  and 
the  ^^^g  divides,  is  still  in  some  measure  a  matter  of  dispute.  In  a 
large  number  of  cases,  however,  it  is  certainly  known  that  tJie  egg-cen- 
trosome  disappears  before  or  during  fertilization  and  its  place  is  take7t 
by  a  new  centjvsome  zvhich  is  introduced  by  the  spermatozoon  and 
divides  into  two  to  form  the  cleavage-amp  Master.  This  has  been 
conclusively  demonstrated  in  several  forms  (various  echinoderms, 
annelids,  nematodes,  tunicates,  mollusks,  and  vertebrates)  and  estab- 
lished with  a  high  degree  of  probability  in  many  others  (insects,  Crus- 
tacea). In  every  accurately  known  case,  moreover,  the  centrosome 
has  been  traced  to  the  middle-piece  of  the  spermatozoon  ;  e.g.  in 
sea-urchins  (Hertwig,  Boveri,  Wilson,  Mathews,  Hill),  in  the  axolotl 
(Fick),  in  the  tunicate  Phallusia  (Hill),  probably  in  the  earthworm, 

1  Cf.  p.  154. 


36 


FERTILIZATION   OF   THE    OVUM 


Allolobophora  (Foot),  in  the  butterfly  Pieris  (Henking),  and  in  the 
gasteropod  Physa  (Kostanecki  and  Wierzejski).  The  agreement 
between  forms  so  diverse  is  very  strong  evidence  that  this  must  be 
regarded  as  the  typical  derivation  of  the  centrosome.^ 

The  facts  may  be  illustrated  by  a  brief    description  of  the    phe- 


99 


fk    •    •    • 


99^ 


1  • 

•     \ 

J 

i'* 

X 

/ 

\  ^- 

^  /\ 

/ 

^# 

"-^ 

-^ 

0 

/ 

*       9 

«• 

.1* 

«' 

•  S® 

96 

c/4- 


D 

Fig.  67.  —  Maturation  and  fertilization  of  the  egg  of  the  mouse.  [Soi^OTTA.] 
A.  The  ovarian  egg  still  surrounded  by  the  follicle-cells  and  the  membrane  {z.p.,  zona  pel- 
lucida) ;  the  polar  spindle  formed.  D.  Egg  immediately  after  entrance  of  the  spermatozoon 
(sperm-nucleus  at  cT).  C  The  two  germ-nuclei  (cf,  $)  still  unequal;  polar  bodies  above. 
D.  Germ-nuclei  approaching,  of  equal  size.  E.  The  chromosomes  forming.  F.  Ihe  minute 
cleavage-spindle  in  the  centre;  on  either  side  the  paternal  and  maternal  groups  of  chromosomes. 

nomena  in  the  sea-urchin  Toxopnenstes  (Fig.  69).  As  described  at 
p.  146,  the  tail  is  in  this  case  left  outside,  and  only  the  head  and 
middle-piece  enter  the  ^g^.  Within  a  few  minutes  after  its  entrance, 
and  while  still  very  near  the  periphery,  the  lance-shaped  sperm-head, 
carrying  the  middle-piece  at  its  base,  rotates  through  nearly  or  quite 

1  Cf.  p.  156. 


GENERAL   SKETCH 


m 


1 80°,  SO  that  the  pointed  end  is  directed  outward  and  the  middle- 
piece  is  turned  inward  (Fig.  69  A-F)}  During  the  rotation  a  minute 
aster  is   developed    about  the    middle-piece  as  a  centre,  and  at  the 


S-" 


€^ 


B 

Fig.  68.  —  Fertilization  of  the  egg  of  the  gasteropod  Pterotrachea.  [BOVERI.] 
A,  The  egg-nucleus  {E)  and  sperm-nucleus  (6")  approaching  after  formation  of  the  polar 
l)odies ;  the  latter  shown  above  {P.B.) ;  each  germ-nucleus  contains  sixteen  chromosomes ;  the 
sperm-ampiiiaster  fully  developed.  B.  The  mitotic  figure  for  the  first  cleavage  nearly  established; 
the  nuclear  membrant^s  have  disappeared  leaving  the  maternal  group  of  chromosomes  above 
the  spindle,  the  paternal  below  it. 


1  The  first,  as  far  as  I  know,  to  observe  the  rotation  of  the  sperm-head  was  Flemming  in 
the  echinoderm-egg  ('8i,  pp.  17-19).  It  has  since  been  clearly  observed  in  several  other 
cases,  and  is  probably  a  phenomenon  of  very  general  occurrence. 


38 


FERTILIZATION  OF  THE    OVUM 


central  point  a  minute  intensely  staining  centrosome  may  be  seen.^ 
As  the  sperm-nucleus  advances,  the  aster  leads  the  way,  and  at  the 
same  time  rapidly  grows,  its  rays  extending  far  out  into  the  cytoplasm 
and  finally  traversing  nearly  an  entire  hemisphere  of  the  Qg'g.  The 
central  mass  of  the  aster  comes  in  contact  with  the  egg-nucleus,  di- 
vides into  two,  and  the  daughter-asters  pass  to  opposite  poles  of  the 
egg-nucleus,  while  the  sperm-nucleus  flattens  against  the  latter  and 
assumes  the  form  of  a  biconvex  lens  (Fig.  70).  The  nuclei  now  fuse 
to  form  the  cleavage-nucleus.  Shortly  afterwards  the  nuclear  mem- 
brane fades  away,  a  spindle  is  developed  between  the  asters,  and 


.^™.. 


.-^^?^>v. 


Fig.  69.  —  Entrance  and  rotation  of  the  sperm-head  and  formation  of  the  sperm-aster  in  the 
sea-urchin  Toxopneustes  {A.-F.,  X  1600;    G,  //.,  X  800). 

A.  Sperm-head  before  entrance;  n,  nucleus;  in,  middle-piece  and  part  of  the  fias;ellum. 
B.  C.  Immediately  after  entrance,  showing  entrance-cone.  D.-F.  Rotation  of  the  sperm-head, 
formation  of  the  sperm-aster  about  the  middle-piece  (the  minute  centrosome  not  shown). 
G.  H.  Approach  of  the  germ-nuclei ;   growth  of  the  aster. 

a  group  of  chromosomes  arises  from  the  cleavage-nucleus.  These 
are  36  or  38  in  number ;  and  although  their  relation  to  the  paternal 
and  maternal  chromatin  cannot  in  this  case  be  accurately  traced, 
owing  to  the  apparent  fusion  of  the  nuclei,  there  can  be  no  doubt  on 
general  grounds  that  one-half  have  been  derived  from  each  germ- 
nucleus.  Throughout  these  changes  no  trace  of  an  egg-centrosome 
is  to  be  discovered.  This  centrosome,  though  present  in  earlier  stages, 
has  been  lost  after  the  polar  bodies  were  formed  by  the  ovarian  ^gg. 

1  I  was  unable  to  find  such  a  centrosome  in  Toxopneustes,  but  the  observations  of  Boveri 
and  Hill  prove  that  it  is  certainly  present  in  other  sea-urchins,  and  I  now  believe  my  own 
account  to  have  been  at  fault  in  this  respect. 


GENERAL   SKETCH 


139 


The  facts  just  described  are  now  known  to  be  typical  of  a  large 
number  of  cases.     We  may,  however,  distinguish  two  types  of  ferti- 


Fig.  70.  —  Conjugation  of  the  germ-nuclei  and  division  of  the  sperm-aster  in  the  sea-urchin 
Toxop/ieustes,  x  1000.     (For  later  stages  see  Fig.  37.) 

.  /.  Union  of  the  nuclei,  extension  of  the  aster.  B.  Flattening  of  the  sperm-nucleus  against  the 
egg-nucleus,  division  of  the  aster. 


lization  according  as  the  polar-bodies  are  formed  before  or  after  the 
entrance  of  the  spermatozoon.  In  the  first  case,  well  illustrated  by 
the  sea-urchin  (Fig.  69),  the  germ-nuclei  conjugate  immediately  after 


140  FERTILIZATION   OF   THE    OVUM 

entrance  of  the  spermatozoon.  In  the  second  and  more  frequent  case 
(Asain's,  Fig.  65  ;  Physa,  Fig.  64;  Nereis,  Fig.  71  ;  Cyclops,  Fig.  72), 
the  sperm-nucleus  penetrates  for  a  certain  distance,  often  to  the  cen- 
tre of  the  ^g%,  and  then  pauses  while  the  polar  bodies  are  formed. 
It  then  conjugates  with  the  reformed  egg-nucleus.  In  this  case, 
the  sperm-aster  always  divides  to  form  an  amphiaster  before  conju- 
gation of  the  nuclei,  while  in  the  first  case  the  aster  may  be  still 
undivided  at  the  time  of  union.  This  difference  is  doubtless  due 
merely  to  a  difference  in  the  time  elapsing  between  entrance  of  the 
spermatozoon  and  conjugation  of  the  nuclei,  the  amphiaster  having, 
in  the  second  case,  time  to  form  during  extrusion  of  the  polar  bodies. 

It  is  an  interesting  and  significant  fact  that  the  aster  or  amphiaster 
always  leads  the  way  in  the  march  towards  the  egg-nucleus ;  and  in 
many  cases  it  may  be  far  in  advance  of  the  sperm-nucleus. ^  Boveri 
{^%%,  i)  has  observed  in  sea-urchins  that  the  sperm-nucleus  may  indeed 
be  left  entirely  behind,  the  aster  alone  conjugating  with  the  egg- 
nucleus  and  causing  division  of  the  ^%^  without  union  of  the  germ- 
nuclei,  though  the  sperm-nucleus  afterwards  conjugates  with  one  of 
the  nuclei  of  the  two-cell  stage.  This  process,  known  as  ''  partial  fer- 
tilization," is  undoubtedly  to  be  regarded  as  abnormal.  It  affords, 
however,  a  beautiful  demonstration  of  the  fact  that  it  is  the  centro- 
soine  alone  that  causes  division  of  the  egg,  and  it  is  therefore  the  fei'- 
tilizing  element  proper  (Boveri,  '^J,  2).  We  may  therefore  conclude 
that  the  end  of  fertilization  is  the  union  of  the  germ-nuclei  and  the 
equal  distribution  of  their  substance,  while  the  active  agent  in  this 
process  is  the  centrosome. 

The  earliest  investigators  of  fertilization,  such  as  Blitschli  and  Fol, 
had  no  knowledge  of  the  centrosome,  and  hence  no  clear  idea  as  to 
the  origin  of  the  asters,  but  Fol  stated  in  1873  that  the  asters  repre- 
sented "centres  of  attraction"  lying  outside  and  independent  of  the 
nucleus.  Oscar  Hertwig  showed,  in  1875,  that  in  the  sea-urchin  ^g^ 
the  amphiaster  arises  by  the  division  of  a  single  aster  that  first 
appears  near  the  sperm-nucleus  and  accompanies  it  in  its  progress 
toward  the  egg-nucleus.  A  similar  observation  was  soon  afterwards 
made  by  Fol  ('79)  in  the  eggs  of  Asterias  and  Sagitta,  and  in  the 
latter  case  he  determined  the  fact  that  the  astral  rays  do  not  centre 
in  the  nucleus,  as  Hertwig  described,  but  at  a  point  in  advance  of  it, 
—  a  fact  afterwards  confirmed  by  Hertwig  himself  and  by  Boveri 
('88,  i).  Hertwig  and  Fol  afterwards  found  that  in  cases  of  poly- 
spermy, when  several  spermatozoa  enter  the  Q.gg,  each  sperm-nucleus 
is  accompanied  by  an  aster,  and  Hertwig  proved  that  each  of  these 
might  give  rise  to  an  amphiaster  (Fig.  75). 

^  Cf.  Kostanccki  and  Wierzejski,  '96. 


GENERAL   SKETCH 


41 


It  was  Boveri  i^^J)  who  first  accurately  traced  the  complete  history 
of  the  centrosome  and  clearly  formulated  the  facts,  proving  that  in 
Ascaris  a  single  centrosome  is  brought  in  by  the  spermatozoon  and 
that  it  divi4es  to  form  two  centres  about  which  are  developed  the  two 


Fig.  71. — Fertilization  of  the  ^g'g  oi  Nereis,  from  sections.  (X  400.) 
A.  Soon 'after  the  entrance  of  the  spermatozoon,  showing  the  minute  sperm-nucleus  at  Q»,  the 
germinal  vesicle  disappearing,  and  the  first  polar  mitotic  figure  forming.  The  empty  spaces  repre- 
sent deutoplasm-spheres  (slightly  swollen  by  the  reagents),  the  firm  circles  oil-drops.  B.  Sperm- 
nucleus  {d)  advancing,  a  minute  amphiaster  in  front  of  it;  first  polar  mitotic  figure  established; 
polar  concentration  of  the  protoplasm.  C.  Later  stage ;  second  polar  body  forming.  D.  The 
))olar  bodies  formed ;  conjugation  of  the  germ-nuclei ;  the  egg-centrosomes  and  asters  have 
disappeared,  leaving  only  the  sperm-amphiaster  (cf.  Fig.  64). 

asters  of  the  cleavage-figure.  He  was  thus  led  to  the  following  con- 
clusion, which  I  believe  still  accurately  expresses  the  truth:  ''The 
ripe  egg  possesses  all  of  the  organs  a7id  qualities  necessary  for  division 
excepting  the  centrosome,  by  zvhich  division  is  initiated.      The  spernia- 


14: 


FERTILIZATION   OF   THE    OVUM 


tozooHy  on  the  other  hand,  is  provided  with  a  centrosojne,  but  lacks  the 
substance  in  which  this  organ  of  division  may  exert  its  activity. 
Through  the  tinion  of  the  tzvo  cells  in  fertilization  all  of  the  essential 
organs  necessary  for  division  are  brought  togetJier  ;  the  egg  now  contains 
a  ccntrosonie  which  by  its  own  division  leads  the  luay  in  the  embryonic 
development}  Boveri  did  not  actually  follow  the  disappearance  of 
the  egg-centrosome,  but  nearly  at  the  same  time  this  process  was 
carefully  described  by  Vejdovsky  in  the  case  of  a  fresh-water  annelid 
Rhynchelmis.     Here,  again,  very  strong  evidence  was  brought  for- 


Fig.  72.  —  Fertilization  of  the  egg  in  the  copepod  Cyclops  strenuus.  [RUCKERT.] 
A.  Sperm-nucleus  soon  after  entrance,  the  sperm-aster  dividing,  B.  The  germ-nuclei  ap- 
proaching; cf,  the  enlarged  sperm-nucleus  with  a  large  aster  at  each  pole;  9,  the  egg-nucleus 
reformed  after  formation  of  the  second  polar  body,  shown  at  the  right.  C.  The  apposed  reticular 
germ-nuclei,  now  of  equal  size;  the  spindle  is  immediately  afterwards  developed  between  the  two 
enormous  sperm-asters ;  polar  body  at  the  left. 

ward  to  show  that  the  cleavage-amphiaster  arises  by  the  division  of 
a  single  sperm-aster.  Very  numerous  observations  to  the  same  effect 
have  been  made  by  later  observers.  Bohm  could  find  in  Petromyzon 
('88)  and  the  trout  ('91)  no  radiations  near  the  egg-nucleus  after  the 
formation  of  the  polar-bodies,  while  a  beautiful  sperm-aster  is  devel- 
oped near  the  sperm-nucleus  and  divides  to  form  the  amphiaster. 
Platner  ('86)  had  already  made  similar  observations  in  the  snail 
Ariott,  and  the  same  result  was  soon  afterwards  reached  by  Braucr 
('92)  in  the  case  of  Braiichipits,  and  by  Julin  ('93)  in  Stylcopsis. 
Pick's  careful  study  of  the  fertilization  of  the  axolotl  ('93)  proved  in 


'87,  2,  p.  155. 


GENERAL   SKETCH  1 43 

a  very  convincing  manner  not  only  that  the  amphiaster  is  a  product 
of  the  sperm-aster,  but  also  that  the  latter  is  developed  about  the 
middle-piece  as  a  centre.  The  same  result  was  indicated  by  Foot's 
observatior^s  on  the  earthworm  ('94),  and  it  was  soon  afterwards 
conclusively  demonstrated  in  echinoderms  through  the  independent 
and  nearly  simultaneous  researches  of  myself  on  the  ^^^  of  Toxo- 
pncHstes,  of  Mathews  on  Arbacia,  and  of  Eoveri  on  Echinus.  Nearly 
at  the  same  time  a  careful  study  was  made  by  Mead  ('95)  of  the 
annelid  Chceopteiiis,  and  of  the  starfish  Asterias  by  Mathews,  both 
observers  independently  showing  that  the  polar  spindle  contains  dis- 
tinct centrosomes,  which,  however,  degenerate  after  the  formation  of 
the  polar  bodies,  their  place  being  taken  by  the  sperm-centrosome, 
which  divides  to  form  an  amphiaster  before  union  of  the  nuclei,  as 
in  RJiyncJielmis.  Exactly  the  same  result  has  since  been  reached  by 
Hill  ('95)  in  SphcBrecliiniis  and  the  tunicate  Phalliisia,  and  by  Kos- 
tanecki  and  Wierzejski  ('96)  in  PJiysa  (Fig.  64)  ;  and  in  all  of  these 
the  centrosome  is  likewise  shown  to  arise  from  the  middle-piece. 
The  origin  of  the  centrosome  from  the  spermatozoon  alone  has  also 
been  shown  by  Riickert  ('95,  2)  in  Cyclops  (Fig.  ^2),  and  is  indicated 
by  Sobotta's  work  ('95)  on  the  fertilization  of  the  mouse  (Fig.  6^). 

Such  an  array  of  evidence,  derived  from  the  study  of  so  many 
diverse  groups,  places  Boveri's  conception  of  fertilization  (p.  141)  on 
a  very  strong  foundation,  and  justifies  the  conclusion  that  the  origin 
of  the  first  cleavage-centrosomes  from  the  spermatozoon  alone  is  a 
phenomenon  of  very  wide,  if  not  of  universal,  occurrence.  The 
descendants  of  these  centrosomes  may  be  traced  continuously  into 
later  cleavage-stages,  and  there  can  be  little  doubt  that  they  are  the 
progenitors  of  all  the  centrosomes  of  the  adult  body.  Boveri  and 
Van  Beneden,  followed  by  a  number  of  later  observers,^  have  followed 
the  daughter-centrosomes  through  every  stage  of  the  first  cleavage 
into  the  blastomeres  of  the  two-cell  stage,  where  they  persist  and  give 
rise  to  the  centrosomes  of  the  four-cell  stage,  and  so  on  in  later  stages. 
This  is  beautifully  shown  in  the  ^^g  of  Thalassema  (Fig.  73),  which 
has  been  carefully  followed  out  in  my  laboratory  by  Mr.  B.  B.  Griffin. 
The  centrosome  is  here  a  minute  granule  at  the  focus  of  the  sperm- 
aster,  which  divides  to  form  an  amphiaster  soon  after  the  entrance  of 
the  spermatozoon.  During  the  early  anaphase  of  the  first  cleavage 
each  centrosome  divides  into  two,  passes  to  the  outer  periphery  of  the 
centrosphere,  and  there  forms  a  minute  amphiaster  for  the  second 
cleavage  before  the  first  cleavage  takes  place  (Fig.  73) !  The  minute 
centrosomes  of  the  second  cleavage  are  therefore  the  direct  descendants 
of  the  sperm-centrosome  ;  and  there  is  good  reason  to  believe  that  the 

^  See  Mead  on  CIuToptcriis.,  '95,  and  Kostanecki  and  Wierzejski  on  Physa,  '96. 


144 


FERrnJZATION   OF   THE    OVUM 


continuity  is  not  broken  in  later  stages.  An  exactly  similar  process 
is  described  by  Kostanecki  and  Wierzejski  in  the  Q.gg  of  PJiysa.  We 
thus  reach  the  following  remarkable  conclusion :  During  cleavage 
the  cytoplasm  of  the  blasto?neres  is  derived  from  that  of  the  egg,  the 
centrosomes  from  the  spermatozoon,  while  the  nuclei  {chromatin)  air 


Fig.  73.  —  Persistence  of  the  centrosomes  from  cell  to  cell,  in  the  cleavage  of  the  egg  of  the 
gephyrean  Thalasseiiia.     [GRIFFIN.] 

A.  Mitotic  figure  for  the  first  cleav;ige ;  the  centrosome  already  double  in  each  centrosphere 
(the  small  black  bodies  are  deutoplasm-spheres).  B.  Early  anaphase ;  migration  of  the  centro- 
somes to  the  periphery  of  the  centrosphere.  C.  Middle  anaphase  (only  one-half  of  the  mitotic 
figure  shown)  ;  daughter-amphiaster  already  formed.  D.  Telophase  ;  the  egg  dividing  and  nuclei 
reforming  ;  the  old  amphiaster  has  disappeared,  leaving  only  the  daughter-amphiaster  in  each  cell. 


UNION  OF   THE    GERM-CELLS  1 45 

equally  derived  from  both  germ-cells  ;  and  certainly  it  would  be  hard 
to  find  more  convincing  evidence  that  the  chromatin  is  the  controlling 
factor  in  the  cell  by  which  its  specific  character  is  determined. 

We  xvo^  proceed  to  a  more  detailed  and  critical  examination  of 
fertilization. 


B.     Union  of  the  Germ-cells 

It  does  not  lie  within  the  scope  of  this  work  to  consider  the 
innumerable  modes  by  which  the  germ-cells  are  brought  together, 
further  than  to  recall  the  fact  that  their  union  may  take  place  inside 
the  body  of  the  mother  or  outside,  ^.nd  that  in  the  latter  case,  both 
eggs  and  spermatozoa  are  as  a  rule  discharged  into  the  water,  where 
fertilization  and  development  take  place.  The  spermatozoa  may 
live  for  a  long  period,  either  before  or  after  their  discharge,  without 
losing  their  fertilizing  power,  and  their  movements  may  continue 
throughout  this  period.  In  many  cases  they  are  motionless  when 
first  discharged,  and  only  begin  their  characteristic  swimming  move- 
ments after  coming  in  contact  with  the  water.  There  is  clear  evi- 
dence of  a  definite  attraction  between  the  germ-cells,  which  is 
in  some  cases  so  marked  (for  example  in  the  polyp  Rettilld)  that 
when  spermatozoa  and  ova  are  mixed  in  a  small  vessel,  each  ovum 
becomes  in  a  few  moments  surrounded  by  a  dense  fringe  of  sperma- 
tozoa attached  to  its  periphery  by  their  heads  and  by  their  move- 
ments actually  causing  the  ovum  to  move  about.  The  nature  of  the 
attraccion  is  not  positively  known,  but  Pfeffer's  researches  on  the 
spermatozoids  of  plants  leave  little  doubt  that  it  is  of  a  chemical 
nature,  since  he  found  the  spermatozoids  of  ferns  and  of  Selaginella 
to  be  as  actively  attracted  by  solutions  of  malic  acid  or  malates  (con- 
tained in  capillary  tubes)  as  by  the  substance  extruded  from  the 
neck  of  the  archegonium.  Those  of  mosses,  on  the  other  hand,  are 
indifferent  to  malic  acid,  but  are  attracted  by  cane-sugar.  These 
experiments  indicate  that  the  specific  attraction  between  the  germ- 
cells  of  the  same  species  is  owing  to  the  presence  of  specific  chemical 
substances  in  each  case.  There  is  clear  evidence,  furthermore,  that 
the  attractive  force  is  not  exerted  by  the  egg-nucleus  alone,  but  by 
the  egg-cytoplasm ;  for,  as  the  Hertwigs  and  others  have  shown, 
spermatozoa  will  readily  enter  egg-fragments  entirely  devoid  of  a 
nucleus. 

In  naked  eggs,  such  as  those  of  some  echinoderms,  and  coelen- 
terates,  the  spermatozoon  may  enter  at  any  point ;  but  there  are 
some  cases  in  which  the  point  of  entrance  is  predetermined  by  the 
presence    of    special    structures    through    which    the    spermatozoon 


146 


FERTILIZATION   OF   THE    OVUM 


enters  (Fig.  74).  Thus,  the  starfish  ^^^g,  according  to  Fol,  pos- 
sesses before  fertilization  a  peculiar  protoplasmic  "  attraction-cone  " 
to  which  the  head  of  the  spermatozoon  becomes  attached,  and  through 
which  it  enters  the  ^^^^  In  some  of  the  hydromedusae,  on  the  other 
hand,  the  entrance  point  is  marked  by  a  funnel-shaped  depression  at 
the  egg-periphery  (Metschnikoff).  When  no  preformed  attraction- 
cone  is  present,  an  "  entrance-cone  "  is  sometimes  formed  by  a  rush 
of  protoplasm  towards  the  point  at  which  the  spermatozoon  strikes 
the  Q,gg  and  there  forming  a  conical  elevation  into  which  the  sperm- 
head  passes.  Jn  the  sea-urchin  (Fig.  74)  this  structure  persists 
only  a  short  time  after  the  spermatozoon  enters,  soon  assuming  a 


/ 


^r^^^^^m-'v.- 


''^:M^M^'  ^ 


Fig.  74.  —  Entrance  of  the  spermatozoon  into  the  egg.  A.-G.  In  the  sea-urchin  Toxopneustes. 
H.  In  the  medusa  Mitrocoma.     [METSCHNIKOFF.]    /.  In  the  star-fish  Asterias.     [FOL.] 

A.  Spermatozoon  of  Toxopneustes,  X  2000;  a,  the  apical  body,  n,  nucleus,  w,  middle-piece, 
/  flagellum.  B.  Contact  with  the  egg-periphery.  C.  D.  Entrance  of  the  head,  formation  of  the 
entrance-cone  and  of  the  vitelline  membrane  {v),  leaving  the  tail  outside.  E.F.  Later  stages, 
G.  Appearance  of  the  sperm-aster  {s)  about  3-5  minutes  after  first  contact ;  entrance-cone  break- 
ing up.  H.  Entrance  of  the  spermatozoon  into  a  preformed  depression.  /.  Approach  of  the 
spermatozoon,  showing  the  preformed  attraction-cone. 


ragged  flame-shape  and  breaking  up  into  slender  rays.  In  some 
cases  the  egg  remains  naked,  even  after  fertilization,  as  appears  to 
be  the  case  in  many  coelenterates.  More  commonly  a  vitelline  mem- 
brane is  quickly  formed  after  contact  of  the  spermatozoon,  —  e.g. 
in  AmpJiioxus,  in  the  echinoderms,  and  in  many  plants,  —  and  by 
means  of  this  the  entrance  of  other  spermatozoa  is  prevented.  In 
eggs  surrounded  by  a  membrane  before  fertilization,  the  spermato- 
zoon either  bores  its  way  through  the  membrane  at  any  point,  as  is 
probably  the  case  with  mammals  and  amphibia,  or  may  make  its 
entrance  through  a  micropyle. 

In  some  forms  only  one  spermatozoon  normally  enters  the  ovum, 


UNION  OF   THE    GERM-CELLS 


47 


as  in  echinoderms,  mammals,  many  annelids,  etc.,  while  in  others 
several  may  enter  (insects,  elasmobranchs,  reptiles,  the  earthworm, 
Petromyzon,  etc.).  In  the  former  case  more  than  one  spermatozoon 
may  acci^Jentally  enter  (pathological  polyspermy),  but  development 
is  then  always  abnormal.  In  such  cases  each  sperm-centrosome 
gives  rise  to  an  amphiaster,  and  the  asters  may  then  unite  to  form 
the  most  complex  polyasters,  the  nodes  of  which  are  formed  by  the 


Fig-  75-  —  Pathological  polyspermy. 
A.  Polyspermy  in  the  egg  oi  Ascaris  ;  below,  the  egg-nucleus  ;  above,  three  entire  spermatozoa 
within  the  egg.     [Sala.] 

D.  Polyspermy  in  sea-urchin,  egg  treated  with  0.005%  nicotine-solution;  ten  sperm-nuclei 
shown,  three  of  which  have  conjugated  with  the  egg-nucleus.  C.  Later  stage  of  an  egg  similarly 
treated,  showing  polyasters  formed  by  union  of  the  sperm-amphiasters.     [O.  and  R.  Hertwig.] 


centrosomes  (Fig.  75).  Such  eggs  either  do  not  divide  at  all  or 
undergo  an  irregular  multiple  cleavage  and  soon  perish.  If,  how- 
ever, only  two  spermatozoa  enter,  the  ^gg  may  develop  for  a  time. 
Thus  Driesch  has  determined  the  interesting  fact,  which  I  have  con- 
firmed, that  sea-urchin  eggs  into  which  two  spermatozoa  have  acci- 
dentally entered  undergo  a  double  cleavage,  dividing  into  four  at  the 
first  cleavage,  and  forming  eight  instead  of  four  micromeres  at  the 


148  FERriLIZATIOiy  OF   THE    OVUM 

fourth  cleavage.  Such  embryos  develop  as  far  as  the  blastula  stage, 
but  never  form  a  gastrula.^  In  cases  where  several  spermatozoa 
normally  enter  the  ^^g  (physiological  polyspermy),  only  one  of  the 
sperm-nuclei  normally  unites  with  the  egg-nucleus,  the  supernumerary 
sperm-nuclei  either  degenerating,  or  in  rare  cases  —  e.g.  in  elasmo- 
branchs  and  reptiles  —  living  for  a  time  and  even  dividing  to  form 
"  merocytes  "  or  accessory  nuclei.  The  fate  of  the  latter  is  still  in 
doubt ;  but  they  certainly  take  no  part  in  fertilization. 

It  is  an  interesting  question  how  the  entrance  of  supernumerary 
spermatozoa  is  prevented  in  normal  monospermic  fertilization.  In 
the  case  of  echinoderm-eggs  Fol  advanced  the  view  that  this  is 
mechanically  effected  by  means  of  the  vitelline  membrane  formed 
instantly  after  the  first  spermatozoon  touches  the  ^gg.  This  is  indi- 
cated by  the  following  facts.  Immature  eggs,  before  the  formation 
of  the  polar  bodies,  have  no  power  to  form  a  vitelline  membrane, 
and  the  spermatozoa  always  enter  them  in  considerable  numbers. 
Polyspermy  also  takes  place,  as  O.  and  R.  Hertwig's  beautiful  ex- 
periments showed  i^^'j),  in  ripe  eggs  whose  vitality  has  been  dimin- 
ished by  the  action  of  dilute  poisons,  such  as  nicotine,  strychnine, 
and  morphine,  or  by  subjection  to  an  abnormally  high  temperature 
(31°  C.) ;  and  in  these  cases  the  vitelline  membrane  is  only  slowly 
formed,  so  that  several  spermatozoa  have  time  to  enter.^  Similar 
mechanical  explanations  have  been  given  in  various  other  cases. 
Thus  Hoffman  believes  that  in  teleosts  the  micropyle  is  blocked  by 
the  polar-bodies  after  the  entrance  of  the  first  spermatozoon ;  and 
Calberla  suggested  {Petroniyzori)  that  the  same  result  might  be 
caused  by  the  tail  of  the  entering  spermatozoon.  It  is,  however, 
far  from  certain  whether  such  rude  mechanical  explanations  are 
adequate ;  and  there  is  considerable  reason  to  believe  that  the  ^gg 
may  possess  a  physiological  power  of  exclusion  called  forth  by  the 
first  spermatozoon.  Thus  Driesch  found  that  spermatozoa  did  not 
enter  fertilized  sea-urchin  eggs  from  which  the  membranes  had  been 
removed  by  shaking.^  In  some  cases  no  membrane  is  formed  (some 
coelenterates),  in  others  several  spermatozoa  are  found  inside  the 
membrane  (nemertines),  in  others  the  spermatozoon  may  penetrate 
the  membrane  at  any  point  (mammals),  yet  monospermy  is  the 
rule. 

1  For  an  account  of  the  internal  changes,  see  p.  261. 

'-^  The  Hertwigs  attribute  this  to  a  diminished  irritabihty  on  the  part  of  the  egg-substance. 
Normally  requiring  the  stimulus  of  only  a  single  spermatozoon  for  the  formation  of  the  vitel- 
line meml)rane,  it  here  demands  the  more  intense  stimulus  of  two,  three,  or  more  before 
the  membrane  is  formed.  That  the  membrane  is  not  present  before  fertilization  is  admitted 
by  Hertwig  on  the  ground  stated  at  \>.  97. 

*  On  the  other  hand,  Morgan  states  ('95,  5,  p.  270)  that  one  or  more  spermatozoa  will 
enter  nucleated  or  enucleated  egg-fragments  whether  obtained  before  or  after  fertilization. 


UNION  OF   THE    GERM-CELLS  1 49 


I.    Immediate  Results  of  Union 

The  union  of  the  germ-cells  calls  forth  profound  changes  in  both. 

{a)  The  Spermatozoon. — Almost  immediately  after  contact  the  tail 
ceases  its  movements.  In  some  cases  the  tail  is  left  outside,  being 
carried  away  on  the  outer  side  of  the  vitelline  membrane,  and  only 
the  head  and  middle-piece  enter  the  ^^g  (echinoderms,  Fig.  74). 
In  other  cases  the  entire  spermatozoon  enters  (amphibia,  earthworm, 
insects,  etc..  Fig.  64),  but  the  tail  always  degenerates  within  the 
ovum  and  takes  no  part  in  fertilization.  Within  the  ovum  the 
sperm-nucleus  rapidly  grows,  and  both  its  structure  and  staining- 
capacity  rapidly  change  (cf.  p.  127).  The  most  important  and  signifi- 
cant result,  however,  is  an  immediate  resumption  by  the  sperm-nucleus 
and  spcrm-centrosome  of  the  power  of  division  which  has  hitherto 
been  suspended.  This  is  not  due  to  the  union  of  the  germ-nuclei ; 
for,  as  the  Hertwigs  and  others  have  shown,  the  supernumerary 
sperm-nuclei  in  polyspermic  eggs  may  divide  freely  without  copu- 
lation with  the  egg-nucleus,  and  they  divide  as  freely  after  entering 
enucleated  egg-fragments.  The  stimulus  to  division  must  therefore 
be  given  by  the  egg-cytoplasm.  It  is  a  very  interesting  fact  that  in 
some  cases  the  cytoplasm  has  this  effect  on  the  sperm-nucleus 
only  after  formation  of  the  polar  bodies  ;  for  when  in  sea-urchins  the 
spermatozoa  enter  immature  eggs,  as  they  freely  do,  they  penetrate 
but  a  short  distance,  and  no  further  change  occurs. 

{b)  The  Ovum. — The  entrance  of  the  spermatozoon  produces  an 
extraordinary  effect  on  the  Qgg,  which  extends  to  every  part  of  its 
organization.  The  rapid  formation  of  the  vitelline  membrane,  already 
described,  proves  that  the  stimulus  extends  almost  instantly  through- 
out the  whole  ovum.^  At  the  same  time  the  physical  consistency  of 
the  cytoplasm  may  greatly  alter,  as  for  instance  in  echinoderm  eggs, 
where,  as  Morgan  has  observed,  the  cytoplasm  assumes  immediately 
after  fertilization  a  peculiar  viscid  character  which  it  afterwards 
loses.  In  many  cases  the  Qgg  contracts,  performs  amoeboid  move- 
ments, or  shows  wave-iike  changes  of  form.  Again,  the  egg-cyto- 
plasm may  show  active  streaming  movements,  as  in  the  formation  of 
the  entrance-cone  in  echinoderms,  or  in  the  flow  of  peripheral  proto- 
plasm towards  the  region  of  entrance  to  form  the  germinal  disc,  as  in 
many  pelagic  fish-eggs.  An  interesting  phenomenon  is  the  formation, 
behind  the  advancing  sperm-nucleus,  of  a  peculiar  funnel-shaped  mass 
of  deeply  staining  material  extending  outwards  to  the  periphery. 
This  has  been  carefully  described  by  Foot  ('94)   in  the  earthworm, 

1  I  have  often  observed  that  the  formation  of  the  membrane,  in  Toxopneiistes.,  proceeds 
like  a  wave  from  the  entrance-point  around  the  periphery,  but  this  is  often  irregular. 


ISO 


FERTILIZATION   OF   THE    OVUM 


where  it  is  very  large  and  conspicuous,  and  I  have  since  observed  it 
also  in  the  sea-urchin  (Fig.  69). 

The  most  profound  change  in  the  ovum  is,  however,  the  migration 
of  the  germinal  vesicle  to  the  periphery,  and  the  formation  of  the 
polar  bodies.  In  many  cases  either  or  both  these  processes  may  occur 
before  contact  with  the  spermatozoon  (echinoderms,  some  vertebrates). 
In  others,  however,  the  ^gg  awaits  the  entrance  of  the  spermatozoon 
(annelids,  gasteropods,  etc.),  which  gives  it  the  necessary  stimulus. 
This  is  well  illustrated  by  the  ^g^  of  Nereis.  In  the  newly-dis- 
charged ^gg   the  germinal  vesicle  occupies  a  central  position,  the 

yolk,  consisting  of  deutoplasm- 
spheres  and  oil-globules,  is  uni- 
formly distributed,  and  at  the 
periphery  of  the  ^gg  is  a  zone  of 
clear  perivitelline  protoplasm  (Fig. 
43).  Soon  after  entrance  of  the 
spermatozoon  the  germinal  vesicle 
moves  towards  the  periphery,  its 
membrane  fades  away,  and  a  radi- 
ally directed  mitotic  figure  appears, 
by  means  of  which  the  first  polar 
body  is  formed  (Fig.  71).  Mean- 
while the  protoplasm  flows  towards 
the  upper  pole,  the  perivitelline 
zone  disappears,  and  the  ^^g  now 
shows  a  sharply  marked  polar 
differentiation.  A  remarkable  phe- 
nomenon, described  by  Whitman 
in  the  leech  ('78),  and  later  by 
Foot  in  the  earthworm  ('94),  is 
the  formation  of  *' polar  rings,"  a  process  which  follows  the  entrance 
of  the  spermatozoon  and  accompanies  the  formation  of  the  polar 
bodies.  These  are  two  ring-shaped  cytoplasmic  masses  which  form 
at  the  periphery  of  the  ^gg  near  either  pole  and  advance  thence 
towards  the  poles,  the  upper  one  surrounding  the  point  at  which  the 
polar  bodies  are  formed  (Fig.  'j6).  Their  meaning  is  unknown,  but 
Foot  ('96)  has  made  the  interesting  discovery  that  they  are  probably 
of  the  same  nature  as  the  yolk-nuclei  (p.  121). 


Fig.  "76.  —  Egg  of  the  leech  Clepsine,  dur- 
ing fertilization.     [WHITMAN.] 

p.b.,  polar  bodies  ;/.r.,  polar  rings  ;  cleav 
age-nucleus  near  the  centre. 


UNION   OF   THE    GERM-CELLS  151 


2.    Paths  of  the  Germ-miclei  (Pro-nicclei)  ^ 

After  the  entrance  of  the  spermatozoon  both  germ-nuclei  move 
through  the  egg-cytoplasm  and  finally  meet  one  another.  The  paths 
traversed  by  each  vary  widely  in  different  forms.  In  general  two 
classes  are  to  be  distinguished,  according  as  the  polar-bodies  are 
formed  before  or  after  entrance  of  the  spermatozoon.  In  the  former 
case  (echinoderms)  the  germ-nuclei  unite  at  once.  In  the  latter  case 
the  sperm-nucleus  advances  a  certain  distance  into  the  ^gg  and  then 
pauses  while  the  germinal  vesicle  moves  towards  the  periphery,  and 
gives  rise  to  the  polar-bodies  {Ascaris,  annelids,  etc.).  This  significant 
fact  proves  that  the  attractive  force  between  the  two  nuclei  is  only 
exerted  after  the  formation  of  the  polar-bodies,  and  hence  that  the 
entrance-path  of  the  sperm-nucleus  is  not  determined  by  such  at- 
traction. A  second  important  point,  first  pointed  out  by  Roux,  is 
that  the  path  of  the  sperm-nucleus  is  cttrved,  its  "  entrance-path " 
into  the  ^gg  forming  a  considerable  angle  with  its  "copulation-path" 
towards  the  egg-nucleus. 

These  facts  are  well  illustrated  in  the  sea-urchin  Qgg  (Fig.  '/'/)y 
where  the  egg-nucleus  occupies  an  eccentric  position  near  the  point 
at  which  the  polar  bodies  are  formed  (before  fertilization).  Entering 
the  egg  at  any  point,  the  sperm-nucleus  first  moves  rapidly  inward 
along  an  entrance-path  that  shows  no  constant  relation  to  the  position 
of  the  egg-nucleus  and  is  approximately  but  never  exactly  radial,  i.e. 
towards  a  point  near  the  centre  of  the  Qgg.  After  penetrating  a 
certain  distance  its  direction  changes  slightly  to  that  of  the  copulation- 
path,  which,  again,  is  directed  not  precisely  towards  the  egg-nucleus, 
but  towards  a  meeting-point  where  it  comes  in  contact  with  the 
egg-nucleus.  The  latter  does  not  begin  to  move  until  the  entrance- 
path  of  the  sperm-nucleus  changes  to  the  copulation-path.  It  then 
begins  to  move  slowly  in  a  somewhat  curved  path  towards  the  meeting- 
point,  often  showing  slight  amoeboid  changes  of  form  as  it  forces  its 
way  through  the  cytoplasm.  From  the  meeting-point  the  apposed 
nuclei  move  slowly  toward  the  point  of  final  fusion,  which  in  this  case 
is  near,  but  never  precisely  at,  the  centre  of  the  egg. 

These  facts  indicate  that  the  paths  of  the  germ-nuclei  are  deter- 

1  The  terms  "female  pro-nucleus,"  "male  pro-nucleus"  (Van  Beneden),  are  often  ap- 
plied to  the  germ-nuclei  before  their  union.  These  should,  I  think,  be  rejected  in  favour  of 
Hertvvig's  terms  egg-nucleus  2iX\d  sperm-nucleus,  on  two  grounds:  (i)  The  germ-nuclei  are 
true  nuclei  in  every  sense,  differing  from  the  somatic-nuclei  only  in  the  reduced  number  of 
<;hromosomes.  As  the  latter  character  has  recently  been  shown  to  be  true  also  of  the 
somatic  nuclei  in  the  sexual  generation  of  plants  (p.  196),  it  cannot  be  made  the  ground  for 
a  special  designation  of  the  germ-nuclei.  (2)  The  germ-nuclei  are  not  male  and  female 
in  any  proper  sense  (p.  183). 


15: 


FERTILIZATION  OF  THE    OVUM 


mined  by  at  least  two  different  factors,  one  of  which  is  an  attraction 
or  other  dynamical  relation  between  the  nuclei  and  the  cytoplasm, 
the  other  an  attraction  between  the  nuclei.     The  former  determines 


Pig-  77-  —  Diagrams  showing  the  paths  of  the  germ-nuclei  in  four  different  eggs  of  the  sea- 
urchin  Toxopiieustes.     From  camera  drawings  of  the  transparent  hving  eggs. 

In  all  the  figures  the  original  position  of  the  egg-nucleus  (reticulated)  is  sliown  at  9  ;  the  point 
at  which  the  spermatozoon  enters  at  E  (entrance-cone).  Arrows  indicate  tlie  paths  traversed  by 
the  nuclei.  At  the  meeting-point  {M)  the  ejjg-nucleus  is  dotted.  The  cleavage-nucleus  in  its 
final  position  is  ruled  in  parallel  lines,  and  through  it  is  drawn  the  axis  of  the  resulting  cleavage- 
figure.  'I'he  axis  of  the  ^gg  is  indicated  by  an  arrow,  the  point  of  which  is  turned  away  from  the 
micromere-pole.  Plane  of  first  cleavage,  passing  near  the  entrance-point,  shown  by  the  curved 
dotted  line. 


the  entrance-path  of  the  sperm-nucleus,  while  both  factors  probably 
operate  in  the  determination  of  the  copulation-path  along  which  it 
travels  to  meet  the  egg-nucleus.  The  real  nature  of  neither  factor 
is  known. 


UNION   OF   THE    GERM-CELLS  1 53 

Hertwig  first  called  attention  to  the  fact  —  which  is  easy  to  observe  in  the  living 
sea-urchin  egg  —  that  the  egg-nucleus  does  not  begin  to  move  until  the  sperm- 
nucleus  has  penetrated  some  distance  into  the  egg  and  the  sperm-aster  has  attained 
a  considerable  size ;  and  Conklin  ('94)  has  suggested  that  the  nuclei  are  passively- 
drawn  togeflier  by  the  formation,  attachment,  and  contraction  of  the  astral  rays. 
While  this  view  has  some  facts  in  its  favour,  it  is,  I  believe,  untenable,  for  many 
reasons,  among  which  may  be  mentioned  the  fact  that  neither  the  actual  paths 
of  the  pro-nuclei  nor  the  arrangement  of  the  rays  support  the  hypothesis ;  nor  does 
it  account  for  the  conjugation  of  nuclei  when  no  astral  rays  are  developed  (as  in 
Protozoa),  or  are  insignificant  as  compared  with  the  nuclei  (as  in  plants).  I  have 
often  observed  in  cases  of  dispermy  in  the  sea-urchin,  that  both  sperm-nuclei  move 
at  an  equal  pace  towards  the  egg-nucleus ;  but  if  one  of  them  meets  the  egg-nucleus 
first,  the  movement  of  the  other  is  immediately  retarded,  and  only  conjugates  with 
the  egg-nucleus,  if  at  all,  after  a  considerable  interval ;  and  in  polyspermy,  the  egg- 
nucleus  rarely  conjugates  with  more  than  two  sperm-nuclei.  Probably,  therefore, 
the  nuclei  are  drawn  together  by  an  actual  attraction  which  is  neutralized  by  union, 
and  their  movements  are  not  improbably  of  a  chemotactic  character. 


3.    Union  of  the  Gerrn-nnclei.      The  Chromosomes 

The  earlier  observers  of  fertilization,  such  as  Auerbach,  Stras- 
burger,  and  Hertwig,  described  the  germ-nuclei  as  undergoing  a  com- 
plete fusion  to  form  the  first  embryonic  nucleus,  termed  by  Hertwig 
the  cleavage-  or  segmentation-nuelens.  As  early  as  1881,  however, 
Mark  clearly  showed  that  in  the  slug  Limax  this  is  not  the  case,  the 
two  nuclei  merely  becoming  apposed  without  actual  fusion.  Two 
years  later  appeared  Van  Beneden's  epoch-making  work  on  Asearis, 
in  which  it  was  shown  not  only  that  the  nuclei  do  not  fuse,  but  that 
they  give  rise  to  two  independent  groups  of  chromosomes  which 
separately  enter  the  equatorial  plate  and  whose  descendants  pass 
separately  into  the  daughter-nuclei.  Later  observations  have  given 
the  strongest  reason  to  believe  that,  as  far  as  the  chromatin  is  con- 
cerned, a  true  fusion  of  the  nuclei  never  takes  place  during  fertiliza- 
tion, and  that  the  paternal  and  maternal  chromatin  7naj/  remain 
separate  and  distinct  in  the  later  stages  of  development  —  possibly 
throughout  life  (p.  219).  In  this  regard  two  general  classes  may  be 
distinguished.  In  one,  exemplified  by  some  echinoderms,  by  AmpJii- 
oxns,  Phallusia,  and  some  other  animals,  the  two  nuclei  meet  each 
other  when  in  the  reticular  form,  and  apparently  fuse  in  such  a  manner 
that  the  chromatin  of  the  resulting  nucleus  shows  no  visible  distinc- 
tion between  the  paternal  and  maternal  moieties.  In  the  other  class, 
which  includes  most  accurately  known  cases,  and  is  typically  repre- 
sented by  Ascaris  (Fig.  65)  and  other  nematodes,  by  Cyclops  {Y\^.  72), 
and  by  Pterotrachea  (Fig.  6%),  the  two  nuclei  do  not  fuse,  but  only 
place  themselves  side  by  side,  and  in  this  position  give  rise  each  to 
its  own  group  of  chromosomes.     On  general  grounds  we  may  confi- 


154 


FERTILIZATION   OF   THE    OVUM 


dently  maintain  that  the  distinction  between  the  two  classes  is  only 
apparent,  and  probably  is  due  to  corresponding  differences  in  the  rate 
of  development  of  the  nuclei,  or  in  the  time  that  elapses  before  their 
union. ^  If  this  time  be  very  short,  as  in  echinoderms,  the  nuclei  unite 
before  the  chromosomes  are  formed.  If  it  be  more  prolonged,  as  in 
Ascaris,  the  chromosome-formation  takes  place  before  union. 

With  a  few  exceptions,  which  are  of  such  a  character  as  not  to 
militate  against  the  rule,  tJie  mimber  of  chromosomes  arising  from  the 
germ-iiuclei  is  always  the  same  iji  both,  and  is  one-half  the  nitmbcr 
characteristic  of  the  tissjie-cells  of  the  species.  By  their  union,  tJierc- 
fore,  the  gernt-niiclei  give  rise  to  an  eqitatorial  plate  containing  the 
typical  nnmber  of  chromosomes.  This  remarkable  discovery  was  first 
made  by  Van  Beneden  in  the  case  of  Ascaris,  where  the  number  of 
chromosomes  derived  from  each  sex  is  either  one  or  two.  It  has 
since  been  extended  to  a  very  large  number  of  animals  and  plants,  a 
partial  list  of  which  follows. 

A  Partial  List  showing  the  Number  of  Chromosomes  Char- 
acteristic OF  THE  Germ-Nuclei  and  Somatic  Nuclei  in 
Various  Plants  and  Animals.^ 


Germ- 
Nuclei, 

Somatic 
Nuclei. 

Name. 

Group. 

Authority. 

I 

2 

Ascaris  megalocephala, 
var.  univalens. 

Nematodes. 

Van  Beneden, 
Boveri. 

2 

4 

Id.,  var.  bivalens. 

yy 

77 

)> 

)> 

Ophryotrocha. 

Annelids. 

Korschelt. 

» 

w 

Styleopsis. 

Tunicates. 

Julin. 

4 

8 

Coronilla. 

Nematodes. 

Carnoy. 

?> 

J) 

Pallavicinia. 

Hepaticae. 

Farmer. 

6 

12 

Spiroptera. 

Nematodes. 

Carnoy. 

?> 

W 

Gryllotalpa. 

Insects. 

vom  Rath. 

?> 

» 

Caloptenus. 

71 

Wilcox. 

w 

V 

^quorea. 

Hydromedusae. 

Hacker. 

8 

i6 

Filaroides. 

Nematodes. 

Carnoy. 

?> 

jj 

Hydrophilus. 

Insects. 

vom  Rath. 

V 

» 

Phallusia. 

Tunicates. 

Hill. 

» 

" 

Li  max. 

Gasteropods. 

vom  Rath. 

?> 

[„] 

Rat. 

Mammals. 

Moore. 

»> 

[„] 

Ox,  guinea-pig,  man. 

?) 

Bardeleben. 

» 

» 

Ceratozamia. 

Cycads. 

Overton. 

V 

77 

Pinus. 

Conifera'. 

Dixon. 

1  Indeed,  Boveri  has  found  that  in  Ascaris  both  modes  occur,  though  the  fusion  of  the 
germ-nuclei  is  exceptional.      (Cf.  p.  216.) 

2  The  above  table  is  compiled  from  papers  both  on  fertilization  and  maturation.      Num- 
bers in  brackets  are  inferred. 


UNION  OF   THE    GERM-CELLS 


155 


Germ- 

Somatic 

Name. 

Group. 

Authority. 

Nuclei. 

Nuclei. 

8 

'^ 

Scilla,  Tiiticum. 

Angiosperms. 

Overton. 

J? 

5J 

Allium. 

« 

Strasburger, 
Guignard. 

9 

18 

Echinus. 

Echinoderms. 

Boveri. 

ij 

)5 

Sagitta. 

Chaetognaths. 

?> 

jj 

?> 

Ascidia. 

Tunicates. 

?> 

II 

[22] 

Allolobophora. 

Annelids. 

Foot. 

II  (12) 

22  (24) 

Cyclops  strenuus. 

Copepods. 

Ruckert. 

12 

24 

„       brevicornis. 

?> 

Hacker. 

„ 

V 

Helix. 

Gasteropods. 

Platner,  vom  Rath. 

?• 

Branchipus. 

Crustacea. 

Brauer. 

?? 

[„] 

Pyrrhocoris. 

Insects. 

Henking. 

J? 

7J 

Sal  mo. 

Teleosts. 

Bohm. 

77 

J? 

Salamandra. 

Amphibia. 

Flemming. 

?j 

,V 

Rana. 

?? 

vom  Rath. 

?> 

J? 

Mouse. 

Mammals. 

Sobotta. 

?> 

?» 

Osmunda. 

Ferns. 

Strasburger. 

7> 

r 

Lilium. 

Angiosperms. 

Strasburger, 
Guignard. 

?? 

V 

Helleborus. 

» 

Strasburger. 

V 

?? 

Leucojum,  Pasonia, 
Aconitum. 

» 

Overton. 

14 

28 

Tiara. 

Hydromedusae. 

Boveri. 

16 

32 

Pterotrachea,  Carinaria, 

Phyllirhoe. 

Gastropods. 

77 

. 

D.l 

Diaptomus,  Heterocope. 

Copepods. 

Ruckert. 

J? 

[.] 

Anomalocera,  Euchaeta. 

?? 

vom  Rath. 

?J 

[.] 

Lumbricus. 

Annelids. 

Calkins. 

18 

36 

Torpedo,  Pristiurus. 

Elasmobranchs. 

Ruckert. 

[18(19)] 

36  (38) 

Toxopneustes. 

Echinoderms. 

Wilson. 

84 

168 

Artemia. 

Crustacea. 

Brauer. 

The  above  data  are  drawn  from  sources  so  diverse  and  show  so 
remarkable  a  uniformity  as  to  establish  the  ^general  law  with  a  very- 
high  degree  of  probability.  The  few  known  exceptions  are  almost 
certainly  apparent  only  and  are  due  to  the  occurrence  of  plurivalent 
chromosomes.  This  is  certainly  the  case  with  Ascaris  (cf.  p.  61). 
It  is  probably  the  case  with  the  gasteropod  Avion,  where,  as  described 
by  Platner,  the  egg-nucleus  gives  rise  to  numerous  chromosomes,  the 
sperm-nucleus  to  two  only ;  the  latter  are,  however,  plurivalent,  for 
Garnault  showed  that  they  break  up  into  smaller  chromatin-bodies, 
and  that  the  germ-nuclei  are  exactly  alike  at  the  time  of  union. ^  We 
may  here  briefly  refer  to  remarkable  recent  observations  by  Riickert 
and  others,  which  seem  to  show  that  not  only  the  paternal  and  mater- 

1  '89,  pp.  10,  33. 


T56  FERTILIZATION  OF   THE    OVUM 

nal  chromatin,  but  also  the  chromosomes,  may  retain  their  individu- 
ality throughout  development.^  Van  Beneden,  the  pioneer  observer  in 
this  direction,  was  unable  to  follow  the  paternal  and  maternal  chro- 
matin beyond  the  first  cleavage-nucleus,  though  he  surmised  that  they 
remained  distinct  in  later  stages  as  well ;  and  Rabl  and  Boveri 
brought  forward  evidence  that  the  chromosomes  did  not  lose  their 
identity,  even  in  the  resting  nucleus.  Riickert  ('95,  3)  and  Hacker 
('95,  i)  have  recently  shown  that  in  Cyclops,  the  paternal  and  mater- 
nal chromatin-groups  not  only  remain  distinctly  separated  during  the 
anaphase,  but  give  rise  to  double  nuclei  in  the  two-cell  stage  (Fig.  105). 
Each  half  again  gives  rise  to  a  separate  group  of  chromosomes  at 
the  second  cleavage,  and  this  is  repeated  at  least  as  far  as  the  blas- 
tula  stage.  Herla  and  Zoja  have  shown  furthermore  that  if  in 
Ascarls  the  ^g^  of  variety  bivalens,  having  two  chromosomes,  be 
fertilized  with  the  spermatozoon  of  variety  tmivalcns  having  one 
chromosome,  the  three  chromosomes  reappear  at  each  cleavage,  at 
least  as  far  as  the  twelve-cell  stage  (Fig.  106);  and  according  to  Zoja, 
the  paternal  chromosome  is  distinguishable  from  the  two  maternal  at 
each  step  by  its  smaller  size.  We  have  thus  what  must  be  reckoned 
as  more  than  a  possibility,  that  every  cell  in  the  body  of  the  child  may 
receive  from  each  parent  not  only  half  of  its  chromatin  substance, 
but  one-half  of  its  chromosomes,  as  distinct  and  individual  descendants 
of  those  of  the  parents. 

C.     Centrosome  and  Archoplasm  in  Fertilization 

We  have  now  finally  to  consider  more  critically  the  history  of  the 
centrosomes  in  fertilization,  already  briefly  reviewed  at  p.  135.  The 
account  there  given  considers  only  the  more  usual  and  typical  history 
of  the  centrosome,  viz.  the  degeneration  of  the  egg-centrosome  and 
the  introduction  of  a  new  centrosome  by  the  spermatozoon.  There  is, 
however,  one  phenomenon  which  indicates  a  priori  the  possibility  that 
other  modes  of  fertilization  may  occur,  namely,  partJienogencsis,  in 
which  the  ^gg  develops  without  fertilization.  In  this  case,  as  Brauer 
('93)  has  clearly  shown  in  Artemia,  the  egg-centrosome  remaining 
after  the  formation  of  the  polar  bodies  does  not  degenerate,  but  divides 
into  two  to  form  the  cleavage-amphiaster.  The  degeneration  of  the 
egg-centrosome  is  therefore  not  a  necessary  or  invariable  phenome- 
non, and  as  a  matter  of  fact  several  accounts  have  been  given  of  its 
persistence  and  active  participation  in  the  process  of  fertilization. 
These  accounts  fall  under  three  categories,  as  follows :  — 

I.    Each  germ-cell  contributes  a  single  centrosome,  one  of  which 

^  CL  p.  219. 


CENTKOSOME   AND   ARCHOPLASM  IN  FERTILIZATION  1 57 

forms  the  centre  of  each  aster  of  the  first  mitotic  figure  (Van  Beneden, 
in  Ascaris,  '83,  '^J,  p.  270). 

2.  Each  germ-cell  contributes  two  centrosomes  (or  one  which  im- 
mediately divides  into  two),  which  conjugate,  paternal  with  maternal, 
to  form  those  of  the  cleavage-amphiaster  (Fol,  in  sea-urchins,  '91  ; 
Guignard,  in  flowering  plants,  '91  ;  Conklin,  in  gasteropods,  '93). 

3.  The  centrosome  is  derived  not  from  the  spermatozoon,  but 
from  the  ^gg  (Wheeler,  in  the  case  of  Myzostoma,  '95). 

The  first  of  these  accounts,  which  rested  rather  on  surmise  than 
on  adequate  observation,  may  probably  be  safely  rejected,  for  it  con- 
tradicts the  universal  law  that  the  centrosome  divides  into  two  before 
cell-division,  and  is  unsupported  by  later  observers  (Meyer,  Erlanger, 
etc.).  The  second  view,  as  embodied  in  the  statements  of  Fol,  Gui- 
gnard, and  Conklin,  demands  fuller  consideration.  All  these  authors 
agree  that  each  germ-cell  contributes  two  centrosomes,  or  one  which 
divides  into  two  during  fertilization.  The  daughter-centrosomes  thus 
formed  conjugate  two  and  two  in  such  a  manner  that  each  of  the 
centrosomes  of  the  cleavage-spindle  is  formed  by  the  union  of  a  cen- 
trosome derived  from  each  germ-cell.  It  is  an  interesting  and  sig- 
nificant fact  that  a  conjugation  of  centrosomes  was  predicted  by 
Rabl  ('89)  on  the  a  priori  ground  that  if  the  centrosome  is  a  perma- 
nent cell-organ,  as  Boveri  and  Van  Beneden  maintain,  then  a  union 
of  germ-cells  must  involve  a  union  not  only  of  nuclei,  but  also  of 
centrosomes.  Unusual  interest  was  therefore  aroused  when  Fol,  in 
1 89 1,  under  the  somewhat  dramatic  title  of  the  "Quadrille  of  Cen- 
tres," described  precisely  such  a  conjugation  of  centrosomes  as  Rabl 
had  predicted.  The  results  of  this  veteran  observer  were  very  posi- 
tively and  specifically  set  forth,  and  were  of  so  logical  and  con- 
sistent a  character  as  to  command  instant  acceptance  on  the  part  of 
many  authorities.  Moreover,  a  precisely  similar  result  was  reached 
through  the  careful  studies,  in  the  same  year,  of  Guignard,  on  the 
lily,  and  of  Conklin  ('93),  on  the  marine  gasteropod  Crepidida,  a 
confirmation  which  seemed  to  place  the  quadrille  on  a  firm  basis. 
Fol's  result  was,  however,  opposed  to  the  earlier  conclusions  of  Boveri 
and  Hertwig,  and  a  careful  re-examination  of  the  fertilization  of  the 
echinoderm  ^gg,  independently  made  in  1894-5  by  Boveri  {Echinus), 
by  myself  {Toxopneustes),  and  Mathews  {Arbacia,  Asterias),  demon- 
strated its  erroneous  character.  In  the  echinoderm,  as  in  so  many 
other  cases,  the  egg-centrosome  disappears.  The  cleavage-amphi- 
aster arises  solely  by  division  of  the  sperm-aster,  and  the  centrosome 
of  the  latter  is  derived  not  from  the  tip  of  the  spermatozoon,  as 
asserted  by  Fol,  but  from  the  middle-piece,  as  already  described. 
The  same  result  has  been  since  reached  by  Hill  and  Erlanger. 
Various  attempts  have  been  made  to  explain  Fol's  results  as  based 


158 


FERTILIZATION  OF   THE    OVUM 


on  double-fertilized  eggs,  on  imperfect  method,  on  a  misinterpreta- 
tion of  the  double  centrosomes  of  the  cleavage-spindle,  yet  they  still 
remain  an  inexplicable  anomaly  of  scientific  literature. 


Fig.  78.  —  Fertilization  of  the  egg  of  the  parasitic  annelid  Myzostoma.  [WHEELER.] 
A.  Soon  after  entrance  of  the  spermatozoon;  the  sperm-nucleus  at  cf  ;  at  9  the  germinal 
vesicle  ;  at  c  the  double  egg-centrosome.  B.  First  polar  body  forming  at  9  ;  n,  the  cast-out  nucle- 
olus or  germinal  spot.  C.  The  polar  bodies  formed  {p.b)  \  germ-nuclei  of  equal  size ;  at  c  the 
persistent  egg-centrosomes.  D.  Approach  of  the  germ-nnclei ;  the  egg-amphiaster  formed.  In 
all  other  known  cases  this  amphiaster  is  derived  from  the  jr/^/-;«-amphiaster. 

Serious  doubt  has  also  been  thrown  on  Conklin's  conclusions  by 
subsequent  research.  Kostanecki  and  Wicrzcjski  ('96)  have  recently 
made  a  very  thorough  study,  by  means  of  serial  sections,  of  the  fertil- 


CENTROSOME  AND  ARCIIOPLASM  IN  FERTILIZATION         I  59 

ization  of  the  gasteropod  Physa,  and  have  reached  exactly  the  same 
result  as  that  obtained  in  the  echinoderms.  Ilere  also  the  egg-centre 
degenerates,  and  its  place  is  taken  by  a  centrosome  brought  in  by 
the  spermatozoon  and  giving  rise  to  a  sperm-amphiaster,  which  per- 
sists as  the  cleavage-amphiaster  (Fig.  64).  A  strong  presumption  is 
thus  'created  that  Conklin  was  in  error;  and  if  this  be  the  case,  the 
last  positive  evidence  of  a  conjugation  of  centrosomes  in  the  animal 
Qgg  disappears.^ 

In  view  of  this  result  we  may  well  hesitate  to  accept  Guignard's 
conclusions  in  the  case  of  flowering  plants.  The  figures  of  this 
author  show  in  the  clearest  manner  four  centrosomes  lying  in  the 
neighbourhood  of  the  apposed. germ-nuclei  (Fig.  80) ;  but  the  conju- 
gation of  these  centrosomes  was  an  inference,  not  an  observed  fact, 
and  has  not  been  confirmed  by  any  subsequent  observer.  Until  such 
confirmation  is  forthcoming  we  must  receive  Guignard's  results  with 
scepticism. 2 

The  third  view,  based  upon  the  single  case  of  Myzostoma  as 
described  by  Wheeler  ('95),  apparently  rests  on  strong  evidence, 
though  its  force  cannot  be  exactly  estimated  until  a  more  detailed 
account  has  been  published.  In  this  case  no  sperm-aster  can  be 
seen  at  any  period,  with  which  is  correlated  the  fact  that  no  middle- 
piece  can  be  made  out  in  the  spermatozoon.  The  egg-centrosome, 
on  the  other  hand,  is  stated  to  persist  after  the  formation  of  the 
second  polar  body,  to  become  double  at  a  very  early  period,  and 
to  give  rise  directly  to  the  cleavage-amphiaster  (Fig.  "jZ).  I  can  find 
no  ground  in  Professor  Wheeler's  paper  to  doubt  the  accuracy  of 
his  conclusions.  Nevertheless,  an  isolated  case,  which  stands  in 
contradiction  to  all  that  is  known  of  other  forms,  must  rest  on  irre- 
fragable evidence  in  order  to  command  acceptance.  Since,  more- 
over, the  case  involves  the  whole  theory  of  fertilization  based  on 
other  animals  (cf.  p.  141),  it  must,  I  think,  await  further  investiga- 
tion. 

1  Richard  Hertwig  has,  however,  recently  published  a  very  interesting  observation  which 
indicates  that  we  may  not  yet  have  fully  fathomed  the  facts  in  the  case  of  echinoderms.  If 
unfertilized  echinoderm-eggs,  after  formation  of  the  polar-bodies,  lie  for  many  hours  in 
water  or  be  treated  with  dilute  poisons  (strychnine),  they  may  form  a  more  or  less  perfectly 
developed  amphiaster,  and  the  nucleus  may  even  make  an  abortive  attempt  at  division.  No 
centrosomes,  however,  could  be  discovered,  even  by  the  most  approved  methods.  This 
remarkable  phenomenon  is  probably  of  the  same  nature  as  the  formation  of  artificial  asters 
observed  by  Morgan  (p.  226),  but  its  meaning  is  not  clear. 

2  Van  der  Slricht,  in  a  recent  paper  on  Atnphioxus  ('95),  is  inclined  to  believe  that  a 
fusion  between  the  egg-centre  and  the  sperm-centre  occurs;  but  the  evidence  is  very  incom- 
plete, and  a  comparison  with  the  case  of  Physa  indicates  that  his  conclusion  cannot  be 
sustained.  The  same  criticism  applies  to  the  earlier  work  of  Blanc  ('91,  '93)  on  the  trout's 
egg. 


i6o 


FERTILIZATION   OF   THE    OVUM 


D.     Fertilization  in  Plants 


The  investigation  of  fertilization  in  the  plants  has  always  lagged 
somewhat  behind  that  of  the  animals,  and  even  at  the  present  time 
our  knowledge  of  it  is  less  complete,  especially  in  regard  to  the 
history  of  the  centrosome  and  the  archoplasmic  structures.  It  is, 
however,  sufficient  to  show  that  the  process  is  here  essentially  of 
the  same  nature  as  in  animals  in  so  far  as  it  involves  a  union  of 

two  germ-nuclei  de- 
rived from  the  two 
respective  sexes. 
Many  early  observers 
from  the  time  of 
Pringsheim  ('55)  on- 
ward described  a  con- 
jugation of  cells  in 
the  lower  plants,  but 
the  union  of  geiiii- 
niiclei,  as  far  as  I  can 
find,  was  first  clearly 
made  out  in  the  flow- 
ering plants  by  Stras- 
burger  in  1877-8,  and 
carefully  described 
by  him  in  1884. 
Schmitz  observed  a 
union  of  the  nuclei 
of  the  conjugating  cells  of  Spirogyra  in  1879,  and  made  similar  obser- 
vations on  other  algae  in  1884.  The  same  has  been  shown  to  be  true 
in  MusciiiecE  and  Pteridophytes  by  Strasburger,  Cambell,  and  others 

(Fig-  79)- 

Up  to  the  present  time,  however,  the  only  thorough  investigation 
of  fertilization  has  been  made  in  the  case  of  the  flowering  plants, 
and  our  knowledge  of  the  process  here  is  due  in  the  first  instance  to 
Strasburger  ('84,  "^%)  and  Guignard  ('91),  supplemented  by  the 
work  of  Belajeff  and  Overton.  The  ovum  or  oosphere  of  the  flower- 
ing plant  is  a  large,  rounded  cell  containing  a  large  nucleus  and 
numerous  minute  colourless  plastids  from  which  arise,  by  division,  the 
plastids  of  the  embryo  (chromatophores,  amyloplasts).  The  ovum 
lies  in  the  "embryo-sac,"  which  represents  morphologically  the  female 
prothallium  or  sexual  generation  of  the  Pteridophyte,  and  is  itself 
embedded  in  the  ovule  within  the  ovary.  The  male  germ-cell  is  here 
non-motile,  and   is   represented  by   a   "generative   nucleus,"  with   a 


Fig.  79.  —  Fertilization  in  Fibular ia.  [Cambell,] 
A.B.  Early  stages  in  the  formation  of  the  spermatozoid. 
B.  The  mature  spermatozoid;  the  nucleus  lies  above  in  the 
spiral  turns ;  below  is  a  cytoplasmic  mass  containing  starch- 
grains  {cf.  the  spermatozoids  of  ferns  and  oi  Marsilia,  Fig.  53). 
D.  Archegonium  during  fertilization.  In  the  centre  the  ovum 
containing  the  apposed  germ-nuclei  (cf,  ?). 


FERTILIZATION  IN  PLANTS 


i6i 


small  quantity  of  cytoplasm  and  two  centrosomes  (Guignard),  lying 
near  the  tip  of  the  pollen-tube  (Fig.  80,  A),  which  is  developed  as  an 
outgrowth  from  the  pollen-grain  and  represents,  with  the  latter,  a 
rudimentary  male  prothallium  or  sexual  generation.     The  formation 


Fig.  80.  — Fertilization  of  the  lily.  [GuiGNARD.] 
A.  The  tip  of  the  pollf  n-tube  entering  the  embryo-sac ;  below,  the  ovum  (nosphere)  with  its 
nucleus  at  ?  and  two  centrosomes;  at  the  tip  of  the  pollen-tube  the  sperm-nucleus  (cf)  with  two 
centrosomes  near  it.  D.  Union  of  the  germ-nuclei.  C.  Later  stage  of  the  same,  showing  the 
asserted  fusion  of  the  centrosomes.  E.  The  first  cleavage-figure  in  the  metaphase.  D.  Early 
anaphase  of  the  same;  precocious  division  of  the  centrosomes. 
M 


1 62  FERTILIZATION  OF   THE    OVUM 

of  the  pollen-tube,  and  its  growth  down  through  the  tissue  of  the 
pistil  to  the  ovule,  was  observed  by  Amici  ('23),  Brogniard  ('26), 
and  Robert  Brown  ('31);  and  in  1833-34  Corda  was  able  to  follow 
its  tip  through  the  micropyle  into  the  ovule. ^ 

Strasburger  ('77-88)  first  demonstrated  the  fact  that  the  generative 
nucleus  carried  at  the  tip  of  the  pollen-tube  enters  the  ovum  and 
unites  with  the  egg-nucleus.  On  the  basis  of  these  observations  he 
reached,  in  1884,  the  same  conclusion  as  Hertwig,  that  the  essential 
phenomena  of  fertilization  is  a  union  of  two  germ-nuclei,  and  that 
the  nucleus  is  the  vehicle  of  hereditary  transmission.  Strasburger 
did  not,  however,  observe  the  centrosome  in  fertilization.  This  was 
accomplished  in  1891  by  Guignard,  who  demonstrated  in  the  case  of 
the  lily  {Lilium  Martagori)  that  the  generative  nucleus  as  it  enters 
the  ^g'g  is  accompanied  by  a  small  quantity  of  cytoplasm  and  by  two 
centrosomes  (Fig.  80).  He  showed  further  that  the  ^gg  also  con- 
tains two  centrosomes;  and  according  to  his  account  the  conjugation 
of  the  nuclei  is  accompanied  by  a  conjugation  of  the  centrosomes,  as 
already  described. 

Guignard  also  first  cleared  up  the  history  of  the  chromosomes, 
reaching  results  closely  in  accord  with  those  of  Van  Beneden  in  the 
case  of  Ascaris.  The  two  germ-nuclei  do  not  actually  fuse,  but 
remain  in  contact,  side  by  side,  and  give  rise  each  to  one-half  the 
chromosomes  of  the  equatorial  plate,  precisely  as  in  animals  (Fig.  80). 
The  number  of  chromosomes  from  each  germ-nucleus  is,  in  the  lily, 
twelve.  The  later  history  is  identical  with  that  of  the  animal  ^gg, 
each  chromosome  splitting  lengthwise,  and  the  halves  passing  to 
opposite  poles  of  the  spindle.  Each  daughter-nucleus  therefore 
receives  an  equal  number  of  chromosomes  from  the  maternal  and 
paternal  germ-nuclei.^ 

As  in  the  case  of  animals  (p.  127),  the  germ-nuclei  of  plants  show 
marked  differences  in  structure  and  staining-reaction  before  their 
union,  though  they  ultimately  become  exactly  equivalent.  Thus, 
according  to  Rosen  ('92,  p.  443),  on  treatment  by  fuchsin-methyl-blue 

1  It  is  interesting  to  note  that  the  botanists  of  the  eighteenth  century  engaged  in  the  same 
fantastic  controversy  regarding  the  origin  of  the  embryo  as  that  of  the  zoologists  of  the 
time.  Moreland  (1703),  followed  by  Etienne  Francois  Geoffroy,  Needham,  and  others, 
pla(;ed  himself  on  the  side  of  Leemvenhoek  and  the  spermatists,  maintaining  that  the  pollen 
supplied  the  embryo  which  entered  the  ovule  through  the  micropyle.  (The  latter  had  been 
described  by  Grew  in  1672.)  It  is  an  interesting  fact  that  even  Schleiden  adopted  a  similar 
view.  On  the  other  hand,  Adanson  (1763)  and  others  maintained  that  the  ovule  contained 
the  germ  which  was  excited  to  development  by  an  aura  or  vapour  emanating  from  the  pollen 
and  entering  through  the  trachea?  of  the  pistil. 

2  Guignard's  observations  on  the  conjugation  of  the  centrosomes  have  already  been  con- 
sidered at  p.  159.  They  stand  at  present  isolated  as  the  only  precise  account  of  the  history 
of  the  centrosomes  in  plant-fertilization,  and  no  general  conclusions  on  this  subject  can 
therefore  at  present  be  drawn. 


CONJUGATION  IN   UNICELLULAR  FORMS  1 63 

the  male  germ-nucleus  of  phanerogams  is  '' cyanophilous,"  the  female 
"  erythrophilous,"  as  described  by  Auerbach  in  animals.  Stras- 
burger,  while  confirming  this  observation  in  some  cases,  finds  the 
reaction  t^  be  inconstant,  though  the  germ-nuclei  usually  show 
marked  differences  in  their  staining-capacity.  These  are  ascribed  by 
Strasburger  ('92,  '94)  to  differences  in  the  conditions  of  nutrition  ;  by 
Zacharias  and  Schwarz  to  corresponding  differences  in  chemical 
composition,  the  male  nucleus  being  in  general  richer  in  nuclein,  and 
the  female  nucleus  poorer.  This  distinction  disappears  during  ferti- 
lization, and  Strasburger  has  observed,  in  the  case  of  gymnosperms 
(after  treatment  with  a  mixture  of  fuchsin-iodine-green)  that  the 
paternal  nucleus,  which  is  at  first  "  cyanophilous,"  becomes  "erythro- 
philous," like  the  egg-nucleus  before  the  pollen-tube  has  reached  the 
^&&-  Within  the  ^^<g  both  stain  exactly  alike.  These  facts  indicate, 
as  Strasburger  insists,  that  the  differences  between  the  germ-nuclei 
of  plants  are  as  in  animals  of  a  temporary  and  non-essential  character. 


E.     Conjugation   in   Unicellular   Forms 

The  conjugation  of  unicellular  organisms  possesses  a  peculiar  inter- 
est, since  it  is  undoubtedly  a  prototype  of  the  union  of  germ-cells 
in  the  multicellular  forms.  Biitschli  and  Minot  long  ago  maintained 
that  cell-divisions  tend  to  run  in  cycles,  each  of  which  begins  and 
ends  with  an  act  of  conjugation.  In  the  higher  forms  the  cells  pro- 
duced in  each  cycle  cohere  to  form  the  multicellular  body ;  in  the 
unicellular  forms  the  cells  separate  as  distinct  individuals,  but  those 
belonging  to  one  cycle  are  collectively  comparable  with  the  multi- 
cellular body.  The  validity  of  this  comparison,  in  a  morphological 
sense,  is  generally  admitted.^  No  process  of  conjugation,  it  is  true,  is 
known  to  occur  in  many  unicellular  and  in  some  multicellular  forms, 
and  the  cyclical  character  of  cell-division  still  remains  sub  jiidice? 
It  is  none  the  less  certain  that  a  key  to  the  fertilization  of  higher 
forms  must  be  sought  in  the  conjugation  of  unicellular  organisms. 

The  difficulties  of  observation  are,  however,  so  great  that  we  are 
as  yet  acquainted  with  only  the  outlines  of  the  process,  and  have  still 
no  very  clear  idea  of  its  finer  details  or  its  physiological  meaning. 
The  phenomena  have  been  most  closely  followed  in  the  Infusoria  by 
Biitschli,  Maupas,  and  Richard  Hertwig,  though  many  valuable  ob- 
servations on  the  conjugation  of  unicellular  plants  have  been  made 
by  De  Bary,  Schmitz,  Klebahn,  and  Overton.  All  these  observers 
have  reached  the  same  general  result  as  that  attained  through  study 
of  the  fertilization  of  the  Qgg ;  namely,  that  an  essential  phenomenon 

1  Cf.  p.  41.  2Cf.  p.  129. 


164 


FERTILIZATION  OF   THE    OVUM 


of  conjugation  is  a  union  of  the  nuclei  of  the  co7tjiigati?tg  ceils. 
Among  the  unicellular  plants  both  the  cell-bodies  and  the  nuclei 
completely  fuse.  Among  animals  this  may  occur ;  but  in  many  of 
the  Infusoria  union  of  the  cell-bodies  is  only  temporary,  and  the  con- 
jugation consists  of  a  mutual  exchange  and  fusion  of  nuclei.     It  is 


Second  fission. 


First  fission,  after  separation. 

Differentiation    of   micro-    and 
macronuclei. 


Separation  of  the  gametes 


>  Division    of    tiie    cleavage-nu- 
cleus. 


Cleavage-nucleus. 

Exchange    and    fusion    of    the 
germ-nuclei. 

Germ-nuclei. 


>  Formation  of  the  polar  bodies 


Union  of  the  gametes. 


Fig.  81.  —  Diagram  showing:  the  history  of  the  micronuclei  during  the  conjugation  of  Para- 
vicecium.     [Modified  from  Maupas.] 

^Y  and  F represent  the  opposed  macro-  and  micronuclei  in  the  two  respective  gametes  ;  circles 
represent  degenerating  nuclei ;  black  dots,  persisting  nuclei. 

impossible  within  the  limits  of   this  work  to   attempt   more   than  a 
sketch  of  the  process  in  a  few  forms. 

We  may  first  consider  the  conjugation  of  Infusoria.  Maupas's 
beautiful  observations  have  shown  that  in  this  group  the  life-history 
of  the  species  runs  in  cycles,  a  long  period  of  multiplication  by  cell- 
division  being  succeeded  by  an  "epidemic  of  conjugation,"  which 
inaugurates  a  new  cycle,  and  is  obviously  comparable  in  its  physio- 


CONJUGATION  IN   UNICELLULAR  FORMS  165 

logical  aspect  with  the  union  of  germ-cells  in  the  Metazoa.  If  conju- 
gation do  not  occur,  the  race  rapidly  degenerates  and  dies  out ;  and 
Maupas  believes  himself  justified  in  the  conclusion  that  conjugation 
counteract^  the  tendency  to  senile  degeneration  and  causes  rejuve- 
nescence, as  maintained  by  Biitschli  and  Minot.^ 

In  Stylonychia  pusiulata,  which  Maupas  followed  continuously  from  the  end  of 
February  until  July,  the  first  conjugation  occurred  on  April  29th,  after  128  bi-parti- 
tions ;  and  the  epidemic  reached  its  height  three  weeks  later,  after  175  bi-partitions. 
The  descendants  cf  individuals  prevented  from  conjugation  died  out  through  "  senile 
degeneracy,"'  after  316  bi-partitions.  Similar  facts  were  observed  in  many  other 
forms.  The  degeneracy  is  manifested  by  a  very  marked  reduction  in  size,  a  partial 
atrophy  of  the  cilia,  and  especially  by  a  more  or  less  complete  degradation  of  the 
unclear  apparatus.  In  Stylonychia piistidata  and  Onychodromns grandis  this  process 
especially  affects  the  micronucleus,  which  atrophies,  and  finally  disappears,  though 
the  animals  still  actively  swim,  and  for  a  time  divide.  Later,  the  macronucleus 
becomes  irregular,  and  sometimes  breaks  up  into  smaller  bodies.  In  other  cases, 
the  degeneration  first  affects  the  macronucleus,  which  may  lose  its  chromatin, 
undergo  fatty  degeneration,  and  may  finally  disappear  altogether  {Stylonychia 
Diytilus)^  after  which  the  micronucleus  soon  degenerates  more  or  less  completely,  and 
the  race  dies.  It  is  a  very  significant  fact  that  towards  the  end  of  the  cycle,  as  the 
nuclei  degenerate,  the  animals  become  incapable  of  taking  food  and  of  growth  ;  and 
it  is  probable,  as  Maupas  points  out,  that  the  degeneration  of  the  cytoplasmic  organs 
is  due  to  disturbances  in  nutrition  caused  by  the  degeneration  of  the  nucleus. 

The  more  essential  phenomena  occurring  during  conjugation  are 
as  follows.  The  Infusoria  possess  two  kinds  of  nuclei,  a  large 
macronucleus  and  one  or  more  small  inicronuclei.  During  conjuga- 
tion the  macronucleus  degenerates  and  disappears,  and  the  micronu- 
cleus alone  is  concerned  in  the  essential  part  of  the  process.  The 
latter  divides  several  times,  one  of  the  products,  the  germ-nucleus, 
conjugating  with  a  corresponding  germ-nucleus  from  the  other  indi- 
vidual, while  the  others  degenerate  as  "  corpuscules  de  rebut."  The 
dual  nucleus  thus  formed,  which  corresponds  with  the  cleavage- 
nucleus  of  the  ovum,  then  gives  rise  by  division  to  both  macronuclei 
and  micronuclei  of  the  offspring  of  the  conjugating  animals  (Fig.  81). 

These  facts  may  be  illustrated  by  the  conjugation  of  Paramoscium 
caudatum,  which  possesses  a  single  macronucleus  and  micronucleus, 
and  in  which  conjugation  is  temporary  and  fertilization  mutual.  The 
two  animals  become  united  by  their  ventral  sides  and  the  macronu- 
cleus of  each  begins  to  degenerate,  while  the  micronucleus  divides 
twice  to  form  four  spindle-shaped  bodies  (Fig.  82,  A,  B).  Three  of 
these  degenerate,  forming  the  "  corpuscules  de  rebut,"  which  play 
no  further  part.  The  fourth  divides  into  two,  one  of  which,  the 
''female  pronucleus,"  remains  in  the  body,  while  the  other,  or  "male 
pronucleus,"  passes  into  the  other  animal  and  fuses  with  the  female 

1  Cf.  p.  129. 


Fig.  82.  —  Conjugation  oi  Paramcecium  caudatum.  [A-C,  after  R.  Hertwk;;  D-A',  after 
MaupaS.]     (The  macronuclei  dotted  in  all  the  figures.) 

A.  Micronuclei  preparing  for  their  first  division.  B.  Second  division.  C.  Third  division : 
three  polar  bodies  or  "  corpuscuies  de  rebut,"  and  one  dividing  germ-nucleus  in  each  animal.  D. 
Exchange  of  the  germ-nuclei.  E.  The  same,  enlarged.  F.  Fusion  of  the  germ-nuclei.  G.  The 
same,  enlarged.  //.  Cleavage-nucleus  (c),  preparing  for  the  first  division.  /.  The  cleavage- 
nucleus  has  divided  twice,  y.  After  three  divisions  of  the  cleavage-nucleus;  macronucleus 
breaking  up.  IC.  Four  of  the  nuclei  enlarging  to  form  new  macronuclei.  The  first  fission  soon 
takes  place. 


CONJUGATION  IN   UNICELLULAR   FORMS 


167 


B 


pronucleus  (Fig.  82,  C-H).  Each  animal  now  contains  a  cleavage- 
nucleus  equally  derived  from  both  the  conjugating  animals,  and  the 
latter  soon  separate.  The  cleavage-nucleus  in  each  divides  three 
times  successively,  and  of  the  eight  resulting  bodies  four  become 
macronuclei  and  four  micronuclei  (Fig.  d>2,  H-K).  By  two  suc- 
ceeding fissions  the  four  macronuclei  are  then  distributed,  one  to  each 
of  the  four  resulting  individuals.  In  some  other  species  the  micro- 
nuclei  are  equally  dis- 
tributed in  like  man- 
ner, but  in  P.  caitda- 
tum  the  process  is 
more  complicated, 
since  three  of  them 
degenerate,  and  the 
fourth  divides  twice 
to  produce  four  new 
micronuclei.  In 
either  case  at  the 
close  of  the  process 
each  of  the  conju- 
gating individuals  has 
given  rise  to  four 
descendants,  each 
containing  a  macro- 
nucleus  and  micro- 
nucleus  derived  from 
the  cleavage-nucleus. 
From  this  time  for- 
ward fission  follows 
fission  in  the  usual 
manner,  both  nuclei 
dividing  at  each  fis- 
sion, until,  after  many 
generations,  conjuga- 


Fig.  83.  —  Conjugation  of  Vorticellids.  [Maupas.] 
A.  Attachment  of  the  small  free-swimming  microgamete  to 
the  large  fixed  macrogamete;  micronucleus  dividing  in  each 
{Carchesium).  B.  Microgamete  containing  eight  micronuclei; 
macrogamete  four  {Vorticelld) .  C.  All  but  one  of  the  micro- 
nuclei  have  degenerated  as  polar  bodies  or  "  corpuscules  de 
rebut."  D.  Each  of  the  micronuclei  of  the  last  stage  has  divided 
into  two  to  form  the  germ-nuclei ;  two  of  these,  one  from  each 
gamete,  have  conjugated  to  form  the  cleavage-nucleus  seen  at 
the  left ;  the  other  two,  at  the  right,  are  degenerating. 


tion  recurs. 

Essentially  similar  facts  have  been  observed  by  Richard  Hertwig 
and  Maupas  in  a  large  number  of  forms.  In  cases  of  permanent 
conjugation,  as  in  Vorticella,  where  a  smaller  microgamete  unites  with 
a  larger  macrogamete,  the  process  is  essentially  the  same,  though  the 
details  are  still  more  complex.  Here  the  germ-nucleus  derived  from 
each  gamete  is  in  the  macrogamete  one-fourth  and  in  the  microgam- 
ete one-eighth  of  the  original  micronucleus  (Fig.  83).  Each  germ- 
nucleus  divides  into  two,  as  usual,  but  one  of  the  products  of  each 
degenerates,  and  the  two  remaining  pronuclei  conjugate  to  form  a 
cleavage-nucleus. 


1 68 


FERriLIZATION  OF   THE    OVUM 


The  facts  just  described  show  a  very  close  parallel  to  those  observed 
in  the  maturation  and  fertilization  of  the  ^gg.  In  both  cases  there 
is  a  union  of  two  similar  nuclei  to  form  a  cleavage-nucleus  or  its 
equivalent,  equally  derived  from  both  gametes,  and  this  is  the  pro- 
genitor of  all  the  nuclei  of  the  daughter-cells  arising  by  subsequent 
divisions.  In  both  cases,  moreover  (if  we  confine  the  comparison 
to  the  egg)  the  original  nucleus  does  not  conjugate  with  its  fellow 
until  it  has  by  division  produced  a  number  of  other  nuclei  all  but 
one  of  which  degenerate.  Maupas  does  not  hesitate  to  compare 
these  degenerating  nuclei  or  "corpuscules  de  rebut"  with  the  polar 
bodies  (p.  175),  and  it  is  a  remarkable  coincidence  that  their  number, 
like  that  of  the  polar  bodies,  is  often  three,  though  this  is  not  always 
the  case. 

A    remarkable    peculiarity    in    the   conjugation    of    the    Infusoria 


B 


C 


Fig.  84.  —  Conjugation  of  Noctiluca.     [Ishikawa.] 
A.  Union  of  the  gametes,  apposition  of  the  nuclei.    D.  Complete  fusion  of  the  gametes.    Above 
and   below  the   apposed   nuclei  are   the   centrosomes.      C.  Cleavage-spindle,  consisting   of  two 
separate  halves. 


is  the  fact  that  tJie  germ-niiclei  unite  wJicn  in  the  form  of  spindles 
or  mitotic  figures.  These  spindles  consist  of  achromatic  fibres,  or 
"archoplasm,"  and  chromosomes,  but  no  asters  or  undoubted  cen- 
trosomes have  been  thus  far  seen  in  them.  During  union  the 
spindles  join  side  by  side  (Fig.  82,  G\  and  this  gives  good  reason 
to  believe  that  the  chromatin  of  the  two  gametes  is  equally  dis- 
tributed to  the  daughter-nuclei  as  in  Metazoa.  In  the  conjugation 
of  some  other  Protozoa  the  nuclei  unite  while  in  the  resting  state ; 
but  very  little  is  known  of  the  process  save  in  the  cystoflagellate 
Noctiluca,  which  has  been  studied  with  some  care  by  Cienkowsky 
and  Ishikawa  (Fig.  84).  Here  the  conjugating  animals  completely 
fuse,  but  the  nuclei  are  merely  apposed  and  give  rise  each  to  one- 
half  of  the  mitotic  figure.  At  either  pole  of  the  spindle  is  a  cen- 
trosome,  the  origin  of  which  remains  undetermined. 

It  is  an  interesting  fact  that  in  Noctiluca,  in  the  Gregarines,  and 
probably  in  some  other  Protozoa,  conjugation  is  followed  by  a  very 


CONJUGATION  IN   UNICELLUIAR  FORMS 


169 


rapid  multiplication  of  the  nucleus  followed  by  a  corresponding  divi- 
sion of  the  cell-body  to  form  "spores,"  which  remain  for  a  time 
closely  aggregated  before  their  liberation.  The  resemblance  of  this 
process  to  the  fertilization  and  subsequent  cleavage  of  the  ovum  is 
particularly  striking. 

The   conjugation    of    unicellular    plants    shows    some   interesting 


Fig.  85.  —  Conjugation  of  Spirogyra.  [OvERTON.] 
A.  Union  of  the  conjugating  cells  {S.  communis).  B.  The  typical,  though  not  invariable, 
mode  of  fusion  in  ^.  Weberi :  the  chromatophore  of  the  "  female  "  cell  breaks  in  the  middle, 
while  that  of  the  "  male  "  cell  passes  into  the  interval.  C.  The  resulting  zygospore  filled  with 
pryrenoids,  before  union  of  the  nuclei.  D.  Zygospore  after  fusion  of  the  nuclei  and  formation 
of  the  membrane. 


features.  Here  the  conjugating  cells  completely  fuse  to  form  a 
''zygospore"  (Figs.  85,  99),  which  as  a  rule  becomes  surrounded  by 
a  thick  membrane,  and,  unlike  the  animal  conjugate,  may  long  remain 
in  a  quiescent  state  before  division.  Not  only  do  the  nuclei  unite, 
but  in  many  cases  the  plastids  also  (chromatophores).  In  Spirogyra 
some  interesting  variations  in  this  regard  have  been  observed.  In 
some  species  De  Bary  has  observed  that  the  long  band-shaped  chro- 
matophores unite  end  to  end  so  that  in  the  zygote  the  paternal  and 


170  FERTILIZATION  OF   THE    OVUM 

maternal  chromatophores  lie  at  opposite  ends.  In  5.  Weberi,  on 
the  other  hand,  Overton  has  found  that  the  single  maternal  chromato- 
phore  breaks  in  two  in  the  middle  and  the  paternal  chromatophore 
is  interpolated  between  the  two  halves,  so  as  to  lie  in  the  middle  of 
the  zygote  (Fig.  85).  It  follows  from  this,  as  De  Vries  has  pointed 
out,  that  the  origin  of  the  chromatophores  in  the  daughter-cells 
differs  in  the  two  species,  for  in  the  former  case  one  receives  a 
maternal,  the  other  a  paternal,  chromatophore,  while  in  the  latter, 
the  chromatophore  of  each  daughter-cell  is  equally  derived  from 
those  of  the  two  gametes.  The  final  result  is,  however,  the  same ; 
for,  in  both  cases,  the  chromatophore  of  the  zygote  divides  in  the 
middle  at  each  ensuing  division.  In  the  first  case,  therefore,  the 
maternal  chromatophore  passes  into  one,  the  paternal  into  the  other, 
of  the  daughter-cells.  In  the  second  case  the  same  result  is  effected 
by  two  succeeding  divisions,  the  two  middle-cells  of  the  four-celled 
band  receiving  paternal,  the  two  end-cells  maternal,  chromatophores. 
In  the  case  of  a  Spirogyi'a  filament  having  a  single  chromatophore 
it  is  therefore  "wholly  immaterial  whether  the  individual  cells  re- 
ceive the  chlorophyll-band  from  the  father  or  the  mother  "  (De  Vries), 
—  a  result  which,  as  Wheeler  has  pointed  out,  is  in  a  measure  analo- 
gous to  that  reached  in  the  case  of  the  centrosome  of  the  animal  ^g'g} 


F.     Summary  and  Conclusion 

All  forms  of  fertilization  involve  a  conjugation  of  cells  by  a 
process  that  is  the  exact  converse  of  cell-division.  In  the  lowest 
forms,  such  as  the  unicellular  algae,  the  conjugating  cells  are,  in  a 
morphological  sense,  precisely  equivalent,  and  conjugation  takes 
place  between  corresponding  elements,  nucleus  uniting  with  nucleus, 
cell-body  with  cell-body,  and  even,  in  some  cases,  plastid  with  plastid. 
Whether  this  is  true  of  the  centrosomes  is  not  known,  but  in  the 
Infusoria  there  is  a  conjugation  of  the  achromatic  spindles  which 
certainly  points  to  a  union  of  the  centrosomes  or  their  equivalents. 
As  we  rise  in  the  scale,  the  conjugating  cells  diverge  more  and  more, 
until  in  the  higher  plants  and  animals  they  differ  widely  not  only 
in  form  and  size,  but  also  in  their  internal  structure,  and  to  such  an 
extent  that  they  are  no  longer  equivalent  either  morphologically  or 
physiologically.  Both  in  animals  and  in  plants  the  paternal  germ- 
cell  loses  most  of  its  cytoplasm,  the  main  bulk  of  which,  and  hence 
the  main  body  of  the  embryo,  is  now  supplied  by  the  o^g^.     But, 

1  De  Vries's  conclusion  is,  however,  not  entirely  certain;  for  it  is  impossible  to  deter- 
mine, save  by  analogy,  whether  the  chromatophores  maintain  their  individuality  in  the 
zygote. 


SUMMARY  AND    CONCLUSION  I/I 

more  than  this,  the  germ-cells  come  to  differ  in  their  morphological 
composition ;  for  in  plants  the  male  germ-cell  loses  its  plastids, 
which  are  supplied  by  the  mother  alone,  while  in  most  if  not  all 
animals  the  ^gg  loses  its  centrosome,  which  is  then  supplied  by  the 
father.  iThe  loss  of  the  centrosome  by  the  ^gg  is,  I  believe,  to  be 
regarded  as  a  provision  to  guard  against  parthenogenesis  and  to 
ensure  amphimixis. 

The  equivalence  of  the  germ-cells  is  tJius  finally  lost.  Only  the 
germ-nuclei  retain  their  primitive  morphological  equivalence.  Hence 
zve  find  the  essential  fact  of  fertilization  and  sexual  reproduction  to 
be  a  union  of  equivalent  nuclei;  and  to  this  all  other  processes  are 
tributary.  The  substance  of  the  germ-nuclei,  giving  rise  to  the 
same  number  of  chromosomes  in  each,  is  equally  distributed  to  the 
daughter-cells  and  probably  to  all  the  cells  of  the  body. 

As  regards  the  most  highly  differentiated  type  of  fertilization  and 
development  we  thus  reach  the  following  conception  :  From  the 
mother  comes  in  the  main  the  cytoplasm  of  the  embryonic  body 
which  is  the  principal  substratum  of  growth  and  differentiation. 
From  both  parents  comes  the  hereditary  basis  or  chromatin  by  which 
these  processes  are  controlled  and  from  which  they  receive  the  spe- 
cific stamp  of  the  race.  From  the  father  comes  the  centrosome  to 
organize  the  machinery  of  mitotic  division  by  which  the  Qgg  splits  up 
into  the  elements  of  the  tissues,  and  by  which  each  of  these  elements 
receives  its  quota  of  the  common  heritage  of  chromatin.  Huxley  hit 
the  mark  two  score  years  ago  when  in  the  words  that  head  this  chap- 
ter he  compared  the  organism  to  a  web  of  which  the  warp  is  derived 
from  the  female  and  woof  from  the  male.  What  has  since  been 
gained  is  the  knowledge  that  this  web  is  to  be  sought  in  the  chro- 
matic substance  of  the  nuclei,  and  that  the  centrosome  is  the  weaver 
at  the  loom. 


LITERATURE   IV 

Van  Beneden,  E.  —  Recherches  sur  la  maturation  de  Toeuf,  la  fecondation  et  la  division 

cellulaire:  Arch.  Biol.,  YV.     1883. 
Van  Beneden  and  Neyt.  —  Nouvelles  recherches  sur  la  fecondation  et  la  division 

mitosique  chez  TAscaride  megalocephale :  Bull.  Acad.  roy.  de  Belgiqtie.;  III.  14, 

No.  8,  1887. 
Boveri,  Th.  —  Uber  den  Anteil  des  Spermatozoon  an  der  Teilung  des  Eies :  Sitz.- 

Ber.  d.  Ges.f.  Morph.  u.  Phys.  in  Munchen,  B.  III.,  Heft  3.     1887. 
Id.  —  Zellenstudien,  II.     1888. 

Id,  —  Befruchtung  :  Merkel  und  Bonnefs  Ergebnisse,  1 .     1891 . 
Id.  —  tjber  das  Verhalten  der  Centrosomen  bei  der   Befruchtung  des  Seeigeleies, 

etc.:    Verhandl.  Phys.  Med.  Ges.  Wtirzburg,  XXIX.      1895. 
Fick,  R.  —  tJber  die  Reifung  und  Befruchtung  des  Axolotleies :   Zeitschr.   Wiss. 


Zodl.,  LVI.  4.     1893. 


1/2  FERTILIZATION   OF   THE    OVUM 

Guignard,  L.  —  Nouvelles  .dtudes  sur  la  fdcondation :    Ann.  d.  Sciences  nat.  Bot.j 

XIV.     1891. 
Hartog,  M.  M.  —  Some   Problems  of  Reproduction,  etc. :  Quart.  Joiirn.  Mic.  Sc/., 

XXXIII.     1891. 
Hertwig,  0.  —  Beitrage  zur  Kenntniss  der  Bildung,  Befruchtung  und  Teilung  des 

tierischen  Eies,  I.  :  Morph.  Jalirb..,  I.     1875. 
Hertwig,  R.  —  Uber  die  Konjugation  der  Infusorien  :  Abh.  d.  bayr.  Akad.  d.  VViss.^ 

II.  CI.  XVll.     1888-89. 
Id.  —  Uber  Befruchtung  und  Konjugation  :    Verh.  deutsch.  Zo'dl.   Ges.  Berlin,  i^()2. 
Kostanecki,  K.    v.,  and   Wierzejski,    A.  —  Uber  das  Verhalten  der  sogen.   achro- 

matischen  Substanzen  im  befruchteten  Ei  {oi Physa)  :  Arch.  mik.  Anat.,  XLVII. 

2.     1896. 
Mark,  E.   L.  —  Maturation,  Fecundation,  and  Segmentation  of  Limax  cainpestris: 

Bull.  Mus.  Comp.  Zo'dl.  Harvard  College,  Cambridge,  Mass.,  VI.     1881. 
Maupas.  —  Le  rejeunissement  karyogamique  chez  les  Cili^s :    Arch.  d.  Zobl.,  2'"® 

serie,  VII.     1889. 
Ruckert,  J.  —  Uber  das  Selbstandigbleiben  der  vaterlichen  und  miitterlichen  Kern- 

substanz  wahrend  der  ersten  Entwicklung  des  befruchteten  Cyclops-Eies  :  Arch. 

mik.  Anat.,  XLV.  3.     1895. 
Strasburger,  E.  —  Neue  Untersuchungen  uber  den  Befruchtungsvorgang  bei   den 

Phanerogamen,  als  Grundlage  fUr  eine  Theorie  der  Zeugung.     Jena,  1884. 
Id.  —  Uber   Kern-  und   Zellteilung  im   Pflanzenreich,  nebst   einem   Anhang  uber 

Befruchtung.     Jena,  1888. 
Vejdovsky,   F.  —  Entvvickelungsgeschichtliche   Untersuchungen,    Heft    i,    Reifung, 

Befruchtung  und  Furchung  des  Rhynchelmis-Eies.     Prag,  1888. 
Wilson,  Edm.  B.  —  Atlas  of  Fertilization  and  Karyokinesis.     New  York,  1895. 


CHAPTER   V 

OOGENESIS   AND   SPERMATOGENESIS.      REDUCTION   OF   THE 

CHROMOSOMES 

"  Es  kommt  also  in  der  Generationenreihe  der  Keimzelle  irgendwo  zu  einer  Reduktion 
der  urspriinglich  vorhandenen  Chromosomenzahl  auf  die  Halfte,  und  diese  Za///.fw-reduk- 
tion  ist  demnach  nicht  etwa  nur  ein  theoretisches  Postulat,  sondern  eine  Thatsache." 

BOVERI.I 

Van  Beneden's  epoch-making  discovery  that  the  nuclei  of  the  con- 
jugating germ-cells  contain  each  one-half  the  number  of  chromosomes 
characteristic  of  the  body-cells  has  now  been  extended  to  so  many 
plants  and  animals  that  it  may  probably  be  regarded  as  a  universal 
law  of  development.  The  process  by  which  the  reduction  in  number 
is  effected,  forms  the  most  essential  part  of  the  phenomena  of  matura- 
tion by  which  the  germ-cells  are  prepared  for  their  union.  No  phe- 
nomena of  cell-life  possess  a  higher  theoretical  interest  than  these. 
For,  on  the  one  hand,  nowhere  in  the  history  of  the  cell  do  we  find  so 
unmistakable  and  striking  an  adaptation  of  means  to  ends  or  one  of 
so  marked  a  prophetic  character,  since  maturation  looks  not  to  the 
present  but  to  the  future  of  the  germ-cells.  On  the  other  band,  the 
chromatin-reduction  suggests  problems  relating  to  the  morphological 
constitution  of  nucleus  and  chromatin  which  have  an  important 
bearing  on  all  theories  of  development,  and  which  now  stand  in 
the  foreground  of  scientific  discussion  among  the  most  debatable 
and  interesting  of  biological  problems. 

It  must  be  said  at  the  outset  that  the  phenomena  of  maturation 
belong  to  one  of  the  most  difficult  fields  of  cytological  research,  and 
one  in  which  we  are  confronted  not  only  by  diametrically  opposing 
theoretical  views,  but  also  by  apparently  contradictory  results  of 
observation. 

Two  fundamentally  different  views  have  been  held  of  the  manner 
in  which  the  reduction  is  effected.  The  earlier  and  simpler  view, 
which  was  somewhat  doubtfully  suggested  by  Boveri  i^'^'J,  i ),  and  has 
been  more  recently  supported  by  Van  Bambeke  ('94)  and  some  others, 

1  Zellesistudien,  III.  p.  62, 
173 


74 


REDUCTION   OF  THE    CHROMOSOMES 


assumed  an  actual  degeneration  or  casting  out  of  half  the  chromo- 
somes during  the  growth  of  the  germ-cells  —  a  simple  and  easily 
intelligible  process.  The  whole  weight  of  the  evidence  now  goes  to 
show,  however,  that  this  view  cannot  be  sustained,  and  that  reduction 
is  effected  by  a  rearra^igenient  and  redistribntioji  of  the  nuclear  sub- 
stance without  loss  of  any  of  its  essential  constituents.  It  is  true 
that  a  large  amount  of  chromatin  is  lost  during  the  growth  of  the 
^^g}  It  is  nevertheless  certain  that  this  loss  is  not  directly  con- 
nected with  the  process  of  reduction  ;  for,  as  Hertwig  and  others 
have  shown,  no  such  loss  occurs  during  spermatogenesis,  and  even 
in  the  oogenesis  the  evidence  is  clear  that  an  explanation  must  be 
sought  in  another  direction.  We  have  advanced  a  certain  distance 
towards  such  an  explanation  and,  indeed,  apparently  have  found  it 


Fig.  86.  —  Formation  of  the  polar  bodies  before  entrance  of  the  spermatozoon,  as  seen  in  the 
living  ovarian  agg  of  the  sea-urchin  Toxopneustes  (x  365). 

A.  Preliminary  change  of  form  in  tlie  germinal  vesicle.  D.  The  first  polar  body  formed,  the 
second  forming.  C.  The  ripe  ^gg,  ready  for  fertilization,  after  formation  of  the  two  polar  bodies 
{p.  b.,  I.  2)  ;  e,  the  egg-nucleus.  In  this  animal  the  second  polar  body  fails  to  divide.  For 
division  of  the  second  polar  body  see  Fig.  64. 

in  a  few  specific  cases.  Yet  when  the  subject  is  regarded  as  a 
whole,  the  admission  must  be  made  that  the  time  has  not  yet  come 
for  an  understanding  of  the  phenomena,  and  the  subject  must  there- 
fore be  treated  in  the  main  from  an  historical  point  of  view. 


A.     General  Outline 


The  general  phenomena  of  maturation  fall  under  two  heads ;  viz. 
oogeftesis,  which  includes  the  formation  and  maturation  of  the  ovum, 
and  spermatogenesis,  comprising  the  corresponding  phenomena  in 
case  of  the  spermatozoon.  Recent  research  has  shown  that  matura- 
tion conforms  to  the  same  type  in  both  sexes,  which  show  as  close  a 
parallel  in  this  regard  as  in  the  later  history  of  the  germ-nuclei.    Stated 

J  Cf.  Figs.  71,  88. 


GENERAL    OUTLINE 


175 


in  the  most  general  terms,  this  parallel  is  as  follows  :i  In  both  sexes 
the  final  reduction  in  the  number  of  chromosomes  is  effected  in  the 
course  of  the  last  two  cell-divisions  by  which  the  definitive  germ-cells 
arise,  each  of  the  four  cells  thus  formed  having  but  half  the  usual 
number  of  chromosomes.  In  the  female  but  one  of  the  four  cells 
forms  the  ''ovum  "  proper,  while  the  other  three,  known  as  t\\Q polar 
bodies,  are  minute,  rudimentary,  and  incapable  of  development  (Figs. 
64,  71,  d>6).  In  the  male,  on  the  other  hand,  all  four  of  the  cells  become 
functional  spermatozoa.  This  difference  between  the  two  sexes  is 
probably  due  to  the  physiological  division  of  labour  between  the  germ- 
cells,  the  spermatozoa  being  motile  and  very  small,  while  the  Qgg 
contains   a  large  amount  of  protoplasm  and  yolk,  out  of  which  the 


Primordial  germ-cell. 


Oogonia. 


Primary  oocyte  or  ovarian  egg. 

Secondary  oocytes  (egg  and 

first  polar  body). 


Division-period  (the  number  of  divi- 
sions is  much  greater). 


Growth-period. 


-  Maturation-period. 


Mature  egg  and  three  polar  bodie 

Fig.  87.  —  Diagram  showing  the  genesis  of  the  tgg.     [After  BOVERI.] 

main  mass  of  the  embryonic  body  is  formed.  In  the  male,  therefore, 
all  of  the  four  cells  may  become  functional ;  in  the  female  the  func- 
tions of  development  have  become  restricted  to  but  one  of  the  four, 
while  the  others  have  become  rudimentary  (cf.  p.  182).  The  polar 
bodies  are  therefore  to  be  regarded  as  abortive  eggs  —  a  view  first  put 
forward  by  Mark  in  1881,  and  ultimately  adopted  by  nearly  all  inves- 
tigators. 


I.    Reduction  in  the  Female.     Formation  of  the  Polar  Bodies 

As  described  in  Chapter  III.,  the  Qgg  arises  by  the  division  of  cells 
descended  from  the  primordial  egg-cells  of  the  maternal  organism,  and 
these    may  be  differentiated  from  the  somatic  cells  at  a  very  early 

1  The  parallel  was  first  clearly  pointed  out  by  Plainer  in  1889,  and  was  brilliantly  demon- 
strated by  Oscar  Hertwig  in  the  following  year. 


1/6  REDUCTION  OF   THE    CHROMOSOMES 

period,  sometimes  even  in  the  cleavage-stages.  As  development  pro- 
ceeds, each  primordial  cell  gives  rise,  by  division  of  the  usual  mitotic 
type,  to  a  number  of  descendants  known  as  odgonia  (Fig.  ^y),  which 
are  the  immediate  predecessors  of  the  ovarian  ^g^.  At  a  certain 
period  these  cease  to  divide.  Each  of  them  then  grows  to  form  an 
ovarian  Qg'g,  its  nucleus  enlarging  to  form  the  germinal  vesicle,  its 
cytoplasm  becoming  more  or  less  laden  with  food-matters  (yolk  or 
deutoplasm),  while  egg-membranes  may  be  formed  around  it.  The 
ovum  may  now  be  termed  the  oocyte  (Boveri)  or  ovarian  ^gg. 

In  this  condition  the  egg-cell  remains  until  near  the  time  of  fertili- 
zation, when  the  process  of  maturation  proper  —  i.e.  the  formation  of 
the  polar  bodies  —  takes  place.  In  some  cases,  e.g.  in  the  sea-urchin, 
the  polar  bodies  are  formed  before  fertilization  while  the  q,^^  is  still 
in  the  ovary.  More  commonly,  as  in  annelids,  gasteropods,  nema- 
todes, they  are  not  formed  until  after  the  spermatozoon  has  made  its 
entrance ;  while  in  a  few  cases  one  polar  body  may  be  formed  before 
fertilization  and  one  afterwards,  as  in  the  lamprey-eel,  the  frog,  and 
AmpJiioxiis.  In  all  these  cases,  the  essential  phenomena  are  the 
same.  Two  minute  cells  are  formed,  one  after  the  other,  near  the 
upper  or  animal  pole  of  the  ovum  (Figs.  71,  ^6)\  and  in  many  cases 
the  first  of  these  divides  into  two  as  the  second  is  formed  (Fig.  64). 

A  group  of  four  cells  thus  arises,  namely,  the  mature  Q,g%,  which 
gives  rise  to  the  embryo,  and  three  small  cells  or  polar  bodies  which 
take  no  part  in  the  further  development,  are  discarded,  and  soon  die 
without  further  change.  The  egg-nucleus  is  now  ready  for  union 
with  the  sperm-nucleus. 

A  study  of  the  nucleus  during  these  changes  brings  out  the  follow- 
ing facts.  During  the  multiplication  of  the  oogonia  the  number  of 
chromosomes  is,  in  some  cases  at  any  rate,  the  same  as  that  occurring 
in  the  division  of  the  somatic  cells,^  and  the  same  number  enters  into 
the  formation  of  the  chromatic  reticulum  of  the  germinal  vesicle. 
During  the  formation  of  the  polar  bodies  this  number  becomes 
reduced  to  one-half,  the  nucleus  of  each  polar  body  and  the  egg- 
nucleus  receiving  the  reduced  number.  In  some  manner,  therefore, 
the  formation  of  the  polar  bodies  is  connected  with  the  process  by 
which  the  reduction  is  effected.  The  precise  nature  of  this  process 
is,  however,  a  matter  which  has  been  certainly  determined  in  only  a 
few  cases. 

We  need  not  here  consider  the  history  of  opinion  on  this  subject 
further  than  to  point  out  that  the  early  observers,  such  as  Purkinje, 
von  Baer,  Bischoff,  had  no  real  understanding  of  the  process  and 
believed  the  germinal  vesicle  to  disappear  at  the  time  of  fertilization. 

^  See,  however,  p.  194. 


GENERAL    OUTLINE 


177 


To  Biitschli  i^J^,)  Hertwig,  and  Giard  i^Jj)  we  owe  the  discovery 
that  the  formation  of  the  polar  bodies  is  through  mitotic  division,  the 
chromosomes  of  the  equatorial  plate  being  derived  from  the  chro- 


E  H 

Fig.  88.  —  Diagrams  showing  the  essential  facts  in  the  maturation  of  the  egg.     The  somatic 
number  of  chromosomes  is  supposed  to  be  four. 

A.  Initial  phase;  two  tetrads  have  been  formed  in  the  germinal  vesicle.  B.  The  two  tetrads 
have  been  drawn  up  about  the  spindle  to  form  the  equatorial  plate  of  the  first  polar  mitotic 
fij^ure.  C,  The  mitotic  figure  has  rotated  into  position,  leaving  the  remains  of  the  germinal 
vesicle  at  g.  v.  D.  Formation  of  the  first  polar  body ;  each  tetrad  divides  into  two  dyads. 
E.  First  polar  body  formed ;  two  dyads  in  it  and  in  the  egg.  F.  Preparation  for  the  second 
division.  G.  Second  polar  body  forming  and  the  first  dividing;  each  dyad  divides  into  two 
single  chromosomes.  H.  Final  result;  three  polar  bodies  and  the  egg-nucleus  (9),  each  con- 
taining two  single  chromosomes  (half  the  somatic  number)  ;  c,  the  egg-centrosome  which  now 
degenerates  and  is  lost. 
N 


1/8 


REDUCTION  OF   THE    CHROMOSOMES 


K 


Fig.  89.  —  Formation  of  the  polar  bodies  in  Ascaris  megalocephala,  var.  bivalens.  [BOVERI.] 
A.  The  egg  with  the  spermatozoon  just  entering  at  cT  ;  the  germinal  vesicle  contains  two  rod- 
shaped  tetrads  (only  one  clearly  shown),  the  number  of  chromosojnes  in  earlier  divisions  having 
been  four.  B.  The  tetrads  seen  in  profile.  C.  The  same  in  end  view.  D.  First  spindle  forming 
(in  this  case  inside  the  germinal  vesicle).  E.  First  polar  spindle.  F.  The  tetrads  dividing. 
G.  First  polar  body  formed,  containing,  like  the  egg,  two  dyads.  H.  I.  The  dyads  rotating  into 
position  for  the  second  division.  J.  The  dyads  dividing.  K.  Each  dyad  has  divided  into  two 
single  chromosomes,  completing  the  reduction.     (For  later  stages  see  Fig.  65.) 


GENERAL    OUTLINE  1 79 

matin  of  the  germinal  vesicle. ^  In  the  formation  of  the  first  polar 
body  the  group  of  chromosomes  splits  into  two  daughter-groups,  and 
this  process  is  immediately  repeated  in  the  formation  of  the  second 
witJiojit  an  intervening  i-etictiiar  resting  stage.  The  egg-nucleus 
therefore  receives,  like  each  of  the  polar  bodies,  one-fourth  of  the 
mass  of  chromatin  derived  from  the  germinal  vesicle. 

But  although  the  formation  of  the  polar  bodies  was  thus  shown  to 
be  a  process  of  true  cell-division,  the  history  of  the  chromosomes  was 
found  to  differ  in  some  very  important  particulars  from  that  of  the 
tissue-cells.  The  essential  facts,  which  were  first  accurately  deter- 
mined by  Boveri  in  Ascaris  i^^y,  i),  are  in  a  typical  case  as  follows 
(Figs.  ^d>,  89)  :  As  the  Qgg  prepares  for  the  formation  of  the  first  polar 
body,  the  chromatin  of  the  germinal  vesicle  groups  itself  in  a  num- 
ber of  masses,  each  of  which  splits  up  into  a  group  of  four  bodies 
united  by  linin-threads  to  form  a  "quadruple  group"  or  tetrad 
(Vierergruppe).  The  number  of  tetrads  is  always  one-half  the  usual 
miniber  of  chromosomes.  Thus  in  Ascaris  {megalocephala,  bivalens) 
the  germinal  vesicle  gives  rise  to  two  tetrads,  the  normal  number  of 
chromosomes  in  the  earlier  divisions  being  four ;  in  the  salamander 
and  the  frog  there  are  twelve  tetrads,  the  somatic  number  of  chro- 
mosomes being  twenty-four  (Fleming,  vom  Rath),  etc.  As  the  first 
polar  body  forms,  each  of  the  tetrads  is  halved  to  form  two  double 
groups,  or  dyads,  one  of  which  remains  in  the  ^gg  while  the  other 
passes  into  the  polar  body.  Both  the  ^g'g  and  the  first  polar  body 
therefore  receive  each  a  number  of  dyads  equal  to  one-half  the  usual 
number  of  chromosomes.  The  ^gg  now  proceeds  at  once  to  the 
formation  of  the  second  polar  body  without  previous  reconstruction 
of  the  nucleus.  Each  dyad  is  halved  to  form  two  single  chromo- 
somes, one  of  which,  again,  remains  in  the  ^gg  while  its  sister  passes 
into  the  polar  body.  Both  the  ^gg  and  the  second  polar  body  accord- 
ingly receive  two  single  chromosomes  (one-half  the  usual  number), 
each  of  which  is  one-fourth  of  an  original  tetrad  group.  From  the 
two  remaining  in  the  ^gg  a  reticular  nucleus,  much  smaller  than  the 
original  germinal  vesicle,  is  now  formed.^ 

Essentially  similar  facts  have  now  been  determined  in  a  consider- 
able number  of  animals,  though  the  form  of  the  tetrads  varies  greatly, 
and  there  are  some  cases  in  which  no  actual  tetrad-formation  has  been 
observed  (apparently  in  the  flowering  plants).     It  is  clear  from  the 

1  The  early  accounts  asserting  the  disappearance  of  the  germinal  vesicle  were  based  on 
the  fact  that  in  many  cases  only  a  small  fraction  of  the  chromatic  network  gives  rise  to 
chromosomes,  the  remainder  disintegrating  and  being  scattered  through  the  yolk. 

2  It  is  nearly  certain  that  the  division  of  the  first  polar  body  (which,  however,  may  be 
omitted)  is  analogous  to  that  by  which  the  second  is  formed,  i.e.  each  of  the  dyads  is 
similarly  halved. 


i8o 


REDUCTION  OF   THE    CHROMOSOMES 


foregoing  account  that  the  numerical  reduction  of  Q:\\xovi\?X\xv-rnasses 
takes  place  before  the  polar  bodies  are  actually  formed,  through  the 
operation  of  forces  which  determine  the  number  of  tetrads  within 
the  germinal  vesicle.  The  numerical  reduction  is  therefore  deter- 
mined in  the  grandmother-cell  of  the  ^g^.  The  actual  divisions  by 
which  the  polar  bodies  are  formed  merely  distribute  the  elements  of 
the  tetrads. 

2.    Reduction  in  the  Male.      Spermatogenesis 

The  researches  of  Platner  ('89),  Boveri,  and  especially  of  Oscar 
Hertwig  ('90,  i)  have  demonstrated  that  reduction  takes  place  in  the 


Primordial  germ-cell. 


Spermatogonia. 


-Division-period  (the  number  of  divi- 
sions is  much  greater). 


Growth-period. 


-  Maturation-period. 


Primary  spermatocyte. 

Secondary  spermatocytes. 
Spermatids. 
Spermatozoa. 
Fig.  90.  —  Diagram  showing  the  genesis  of  the  spermatozoon.     [After  BOVERI.] 

male  in  a  manner  almost  precisely  parallel  to  that  occurring  in  the 
female.  Platner  first  suggested  ('89)  that  the  formation  of  the  polar 
bodies  is  directly  comparable  to  the  last  two  divisions  of  the  sperm 
mother-cells  (spermatocytes).  In  the  following  year  Boveri  reached 
the  same  result  in  Ascaris,  stating  his  conclusion  that  reduction  in 
the  male  must  take  place  in  the  "  grandmother-cell  of  the  sperma- 
tozoon, just  as  in  the  female  it  takes  place  in  the  grandmother-cell 
of  the  Qgg,"  and  that  the  egg-formation  and  sperm-formation  really 
agree  down  to  the  smallest  detail  ('90,  p.  64).  Later  in  the  same 
year  appeared  Oscar  Hertwig's  splendid  work  on  the  spermato- 
genesis of  Ascaris,  which  established  this  conclusion  in  the  mo.st 
striking  manner.  Like  the  ova,  the  spermatozoa  are  descended  from 
primordial    germ-cells    which    by   mitotic    division   give    rise   to    the 


GENERAL    OUTLINE 


8r 


spermatogonia  from  which  the  spermatozoa  are  ultimately  formed 
(Fig.  90).  Like  the  oogonia,  the  spermatogonia  continue  for  a  time 
to  divide  with  the  usual  (somatic)  number  of  chromosomes ;  i.e.  four 
in  Ascaris  piegalocephala  bivalens.     Ceasing  for  a  time  to  divide,  they 


Fig.  91. —  Diagrams  sliowing  the  essential  facts  of  reduction  in  the  male.  'Ihe  somatic  num- 
ber ut  chromosomes  is  supposed  to  be  four. 

.  /.  B.  Division  of  one  of  the  spermatogonia,  showing  the  full  number  (four)  of  chromosomes. 
C.  Primary  spermatocyte  preparing  for  division  ;  the  chromatin  forms  two  tetrads.  D.  E.  F.  First 
division  to  form  two  secondary  spermatocytes  each  of  which  receives  two  dyads.  G.  H.  Division 
of  the  two  secondary  spermatocytes  to  form  four  spermatids.  Each  of  the  latter  receives  two 
single  chromosomes  and  a  centrosome  which  persists  in  the  middle-piece  of  the  spermatozoon. 

now  enlarge  considerably  to  form  spermatocytes,  each  of  which  is 
morphologically  equivalent  to  an  unripe  ovarian  ovum,  or  oocyte. 
Each  spermatocyte  finally  divides  twice  in  rapid  succession,  giving 
rise  first  to  two  daughter-spermatocytes  and  then  to  four  spermatids, 
each  of  which  is  directly  converted  into  a  single  spermatozoon.      The 


1 82  REDUCTIOiV  OF   THE    CHROMOSOMES 

history  of  the  chromatin  in  these  tivo  divisions  is  exactly  parallel  to 
that  in  the  formatioti  of  the  polar  bodies  (Figs.  91,  92).  From  the 
chromatin  of  the  spermatocyte  are  formed  a  number  of  tetrads  equal 
to  one-half  the  usual  number  of  chromosomes.  Each  tetrad  is  halved 
at  the  first  division  to  form  two  dyads  which  pass  into  the  respec- 
tive daughter-spermatocytes.  At  the  ensuing  division,  which  occurs 
without  the  previous  formation  of  a  resting  reticular  nucleus,  each 
dyad  is  halved  to  form  two  single  chromosomes  which  enter  the 
respective  spermatids  (ultimately  spermatozoa).  From  each  sperma- 
tocyte, therefore,  arise  four  spermatozoa,  and  each  sperm-nucleus 
receives  half  the  usual  number  of  single  chromosomes.  The  par- 
allel with  the  egg-reduction  is  complete. 

These  facts  leavQ  no  doubt  that  the  spermatocyte  is  the  morpho- 
logical equivalent  of  the  oocyte  or  immature  ovarian  ^^g,  and  that 
the  group  of  four  spermatozoa  to  which  it  gives  rise  is  equivalent 
to  the  ripe  ^gg  plus  the  three  polar  bodies.  Hertwig  was  thus  led  to 
the  following  beautifully  clear  and  simple  conclusion :  "■  The  polar 
bodies  are  abortive  eggs  which  are  formed  by  a  final  process  of 
division  from  the  egg-mother-cell  (oocyte)  in  the  same  manner  as 
the  spermatozoa  are  formed  from  the  sperm-mother-cell  (sperma- 
tocyte). But  while  in  the  latter  case  the  products  of  the  division 
are  all  used  as  functional  spermatozoa,  in  the  former  case  one  of  the 
products  of  the  egg-mother-cell  becomes  the  ^gg,  appropriating  to 
itself  the  entire  mass  of  the  yolk  at  the  cost  of  the  others  which 
persist  in  rudimentary  form  as  the  polar  bodies."  ^ 

3.    Theoretical  Significance  of  Maturation 

Up  to  this  point  the  facts  are  clear  and  intelligible.  When,  how- 
ever, we  attempt  a  more  searching  analysis  by  considering  the  origin 
of  the  tetrads  and  the  ultimate  meaning  of  reduction,  we  find  our- 
selves in  a  labyrinth  of  conflicting  observations  and  hypotheses  from 
which  no  exit  has  as  yet  been  discovered.  And  we  may  in  this  case 
most  readily  approach  the  subject  by  considering  its  theoretical 
aspect  at  the  outset. 

The  process  of  reduction  is  very  obviously  a  provision  to  hold  con- 
stant the  number  of  chromosomes  characteristic  of  the  species ;  for 
if  it  did  not  occur,  the  number  would  be  doubled  in  each  succeeding 
generation  through  union  of  the  germ-cells.  But  why  should  the 
number  be  constant } 

In  its  modern  form  this  problem  was  first  attacked  by  Weismann 
in   1885,  and  again  in   1887,  though  many  earlier  hypotheses  regard- 

1  '90,  I,  p.  126. 


GENERAL    OUTLINE  1 83 

ing  the  meaning  of  the  polar  bodies  had  been  put  forward.^  His 
interpretation  was  based  on  a  remarkable  paper  published  by  Wil- 
helm  Roux  in  1883,^  in  which  are  developed  certain  ideas  which 
afterward^  formed  the  foundation  of  Weismann's  whole  theory  of  in- 
heritance and  development.  Roux  argued  that  the  facts  of  mitosis 
are  only  explicable  under  the  assumption  that  chromatin  is  not  a 
uniform  and  homogeneous  substance,  but  differs  qualitatively  in  differ- 
ent regions  of  the  nucleus  ;  that  the  collection  of  the  chromatin  into  a 
thread  and  its  accurate  division  into  two  halves  is  meaningless  unless 
the  chromatin  in  different  regions  of  the  thread  represents  different 
qualities  which  are  to  be  divided  and  distributed  to  the  daughter- 
cells  according  to  some  definite  law.  He  urged  that  if  the  chromatin 
were  qualitatively  the  same  'throughout  the  nucleus,  direct  division 
would  be  as  efficacious  as  indirect,  and  the  complicated  apparatus  of 
mitosis  would  be  superfluous.  Roux  and  Weismann,  each  in  his  own 
way,  subsequently  elaborated  this  conception  to  a  complete  theory  of 
inheritance  and  development,  but  at  this  point  we  may  confine  our 
attention  to  the  views  of  Weismann.  The  starting-point  of  his  theory 
is  the  hypothesis  of  De  Vries  that  the  chromatin  is  a  congeries  or 
colony  of  invisible  self-propagating  vital  units  or  biophores  somewhat 
like  Darwin's  '' gemmules  "  (p.  303),  each  of  which  has  the  power  of 
determining  the  development  of  a  particular  quality.  Weismann 
conceives  these  units  as  aggregated  to  form  units  of  a  higher 
order  known  as  ''determinants,"  which  in  turn  are  grouped  to  form 

1  Of  these  we  need  only  consider  at  this  point  the  very  interesting  suggestion  of  Minot 
('77),  afterwards  adopted  by  Van  Beneden  ('83),  that  the  ordinary  cell  is  hermaphrodite, 
and  that  maturation  is  for  the  purpose  of  producing  a  unisexual  germ-cell  by  dividing 
the  mother-cell  into  its  sexual  constituents,  or  "  genoblasts."  Thus,  the  male  element  is 
removed  from  the  egg  in  the  polar  bodies,  leaving  the  mature  egg  a  female.  In  like  manner 
he  believed  the  female  element  to  be  cast  out  during  spermatogenesis  (in  the  "  Sertoli 
cells"),  thus  rendering  the  spermatozoa  male.  By  the  union  of  the  germ-cells  in  fertiliza- 
tion the  male  and  female  elements  are  brought  together  so  that  the  fertilized  egg  or  oosperm 
is  again  hermaphrodite  or  neuter.  This  ingenious  view  was  independently  advocated  by 
Van  Beneden  in  his  great  work  on  Ascaris  ('83).  A  fatal  objection  to  it,  on  which  both 
Strasburger  and  Weismann  have  insisted,  lies  in  the  fact  that  male  as  well  as  female  quali- 
ties are  transmitted  by  the  egg-cell,  while  the  sperm-cell  also  transmits  female  qualities. 
The  germ-cells  are  therefore  non-sexual;  they  are  physiologically  as  well  as  morphologi- 
cally equivalent.  The  researches  of  Hertvvig,  Brauer,  and  Boveri  show,  moreover,  that  in 
Ascaris,  at  any  rate,  all  of  the  four  spermatids  derived  from  a  spermatocyte  become  func- 
tional spermatozoa,  and  the  beautiful  parallel  between  spermatogenesis  and  oogenesis  thus 
established  becomes  meaningless  under  Minot's  view.  This  hypothesis  must,  therefore,  in 
my  opinion,  be  abandoned. 

Balfour  probably  stated  the  exact  truth  when  he  said,  "  In  the  formation  of  the  polar 
cells  part  of  the  constituents  of  the  germinal  vesicle,  which  are  requisite  for  its  functions 
as  a  complete  and  independent  nucleus,  is  removed  to  make  room  for  the  supply  of  the 
-necessary  parts  to  it  again  by  the  spermatic  nucleus"  ('80,  p.  62).  He  fell,  however,  into 
the  same  error  as  Minot  and  Van  Beneden  in  characterizing  the  germ-nuclei  as  "  male  " 
and  "  female." 

^  Uber  die  Bedeutung  der  Kerntheihingsfiguren. 


1 84 


REDUCTION  OF   THE    CHROMOSOMES 


"ids,"  the  latter  being  identified  with  the  visible  chromomeres  or 
chromatin-granules.  The  ids  finally  are  associated  in  linear  groups 
to  form  the  **  idants  "  or  chromosomes.  Since  the  biophores  differ 
qualitatively,  it  follows  that  the  same  must  be  true  of  the  higher  units 


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Fig.  92.  —  Reduction  in  the  spermatogenesis  of  Ascaris  megahcephala,  var.  blvalens.  [Brauer.]  ^ 
A-G.  Successive  stages  in  the  division  of  the  primary  spermatocyte.  The  original  reticulum 
undergoes  a  very  early  division  of  the  chromatin-granules  which  then  form  a  doubly  split  spireme- 
thread,  B.  This  shortens  (C),  and  breaks  in  two  to  form  the  two  tetrads  {D  in  profile,  E  viewed 
endwise).  F.  G.  H.  First  division  to  form  two  secondary  spermatocytes,  each  receiving  two  dyads. 
/.  Secondary  spermatocyte.  J.  K.  The  same  dividing.  L.  Two  resulting  spermatids,  each  with 
two  single  chromosomes  and  a  centrosome. 


formed  by  their  aggregation.  Hence  each  chromosome  has  a  dis- 
tinct and  definite  character  of  its  own,  representing  a  particular  group 
of   hereditary  qualities.     From   this  it   follows   that   the   number   of 

1  For  division  of  the  spermatogonia  sec  V\<g.  39  ;    for  the  corresponding  phenomena  in 
var.  tmivaleits  see  Fig.  107. 


GENERAL    OUTLINE  1 85 

specifically  distinct  chromosomes  is  doubled  by  the  union  of  two 
germ-cells,  a  process  which  if  unchecked  would  quickly  lead  to  an 
infinite  complexity  of  the  chromatin  or  germ-plasm.  The  end  of 
maturatior^j  or  reduction,  is  therefore  to  prevent  "  the  excessive 
accumulation  of  different  kinds  of  hereditary  tendencies  or  germ- 
plasms  "  ^  through  the  progressive  summation  of  ancestral  chromatins. 

We  now  come  to  the  vital  point  of  Weismann's  hypothesis  of 
reduction,  about  which  all  later  researches  have  revolved.  Assuming 
with  Roux  that  the  different  qualities  or  ''ancestral  germ-plasms" 
are  arranged  in  a  linear  manner  in  the  spireme-tl;iread  and  in  the 
chromosomes  derived  from  it,  he  ventured  the  prediction  i^^y^  that 
two  kinds  of  mitosis  would  be  found  to  occur.  The  first  of  these 
is  characterized  by  a  longitudinal  splitting  of  the  thread,  as  in  ordi- 
nary cell-division,  "by  means  of  which  all  the  ancestral  germ-plasms 
are  equally  distributed  in  each  of  the  daughter-nuclei  after  having 
been  divided  into  halves."  This  form  of  division,  which  he  called 
"equal  division  "  (Aequationstheilung),  was  then  a  known  fact.  The 
second  form,  at  that  time  a  purely  theoretical  postulate,  he  assumed 
to  be  of  such  a  character  that  each  daughter-nucleus  should  receive 
only  half  the  number  of  ancestral  germ-plasms  possessed  by  the 
mother-nucleus.  This  he  termed  a  "reducing  division"  (Reduk- 
tionstheilung),  and  suggested  ^  that  this  might  be  effected  either  by  a 
transverse  division  of  the  chromosomes,  or  by  the  divergence  and 
separation  of  entire  chromosomes  without  division.  By  either  method 
the  number  of  "  ids "  would  be  reduced ;  and  Weismann  argued 
that  such  reducing  divisions  must  be  involved  in  the  formation  of 
the  polar  bodies,  and  in  the  parallel  phenomena  of  spermatogenesis. 

The  fulfilment  of  Weismann's  prediction  is  one  of  the  most  inter- 
esting results  of  recent  cytological  research.  It  has  been  demon- 
strated, in  a  manner  which  I  believe  is  incontrovertible,  that  the 
reducing  divisions  postulated  by  Weismann  actually  occur,  though 
not  precisely  in  the  manner  conceived  by  him.  Unfortunately,  how- 
ever, this  demonstration  has  been  made  in  only  a  few  specific  cases, 
—  the  complete  demonstration,  indeed,  in  but  a  single  group,  namely, 
the  copepod  Crustacea, —  while  careful  studies  by  the  most  accom- 
plished observers  have  led  to  an  entirely  different  result  in  other 
cases;  namely,  in  Ascaris  and  the  flowering  plants.  We  are  in  fact 
confronted  by  an  apparent  contradiction  of  so  absolute  a  character 
that  no  middle  ground  between  the  conflicting  results  can  at  present 
be  discovered.  We  may  best  appreciate  the  nature  of  this  contra- 
diction by  a  preliminary  consideration  of  the  tetrad  groups  ;  for  it 
is  plain  that  the  nature  of  the  maturation-divisions  can  only  be 
approached  through  a  study  of  the  origin  of  the  tetrads. 

1  Essay  VI.,  p.  366.  ^  I.e.,  p.  375. 


1 86  REDUCTION   OF  THE    CHROMOSOMES 

B.     Origin  of  the  Tetrads 
I.    General  Sketch 

It  is  generally  agreed  that  each  tetrad  arises  by  a  double  division  of 
a  single  primary  chromatin-rod.  Nearly  all  observers  agree  further 
that  the  number  of  primary  rods  at  their  first  appearance  in  the 
germinal  vesicle  or  in  the  spermatocyte-nucleus  is  one-Jialf  the  visual 
number  of  chro7nosomes,  and  that  this  numerical  reduction  is  due  to 
the  fact  that  the  spireme-thread  segments  into  one-half  the  usual  num- 
ber of  pieces.  The  contradiction  relates  to  the  manner  in  which  the 
primary  rod  divides  to  form  the  tetrad.  According  to  one  account, 
mainly  based  on  the  study  of  Ascaris  by  Boveri,  Hertwig,  and  Brauer, 
and  supported  in  principle  by  the  observations  of  Guignard  and 
Strasburger  on  the  flowering  plants,  each  tetrad  arises  by  a  double 
longitudinal  splitting  of  the  primaij  chromatin-rod  caused  by  the 
division  of  each  chromatin-granule  into  four  parts.  In  this  case  the 
four  resulting  bodies  —  i.e.  the  four  chromosomes  of  the  tetrad  — 
must  be  exactly  equivalent,  since  all  are  derived  from  the  same 
region  of  the  spireme-thread  and  consist  of  equivalent  groups  of 
ids  or  chromatin-granules  (Fig.  102,  A).  No  reducing  division  can 
therefore  occur  in  Weismann's  sense.  There  is  only  a  reduction  in 
the  number  of  chromosomes,  not  a  reduction  in  the  number  of  qualities 
represented  by  the  chromatin-granules.  This  may  be  graphically 
expressed  as  follows:  — 

If  the  original  spireme-thread  be  represented  by  abed,  normal 
mitosis    consists    in    its    segmentation    into    the    four    chromosomes 

a  —  b — c  —  d,  which  split  lengthwise  to  form   ->   -r>   -»   -•     In  matu- 

a    0    c    d 

ration  the  thread  segments  into  tivo  portions,  ab  —  cd,  each  of  which 

then  split  into  four  equivalent  portions,  giving  the  equivalent  tetrads, 

y 


ab 

thus,   — r 

ab 


ab         _.    cd 
ab  cd 


cd  X 

— -.J     or   — 
cd  X 


y 

— ,   since    it    is    not     known 

y 


y 

whether  ab  really  is  equal  to  a  -{-  b. 

The  second  account,  which  finds  its  strongest  support  in  the 
observations  of  Riickert,  Hacker,  and  vom  Rath  on  the  maturation  of 
arthropods,  asserts  that  each  tetrad  arises  by  one  longitudinal  and  one 
transverse  diznsion  of  each  primary  chromatin-i^od  {¥\g.  102,  B).  Thus 
the  spireme  abed  segments  as  before  into  two  segments  ab  and  cd. 

These  first  divide  longitudinally  to  form  —  and   ~  and  then  trans- 

ab  cd 


vcrsely  to  form  — ;—    and  - 
a\b  c 


~.     Each  tetrad  therefore  consists,  not  of 
d 


ORIGIN  OF   THE    TETRADS  1 8/ 

four  equivalent  chromosomes,  but  of  two  different  pairs ;  and  the 
second  or  transverse  division  by  which  a  is  separated  from  b,  or  c 
from  dy  is  the  reducing  division  demanded  by  Weismann's  hypoth- 
esis. The/observations  of  Riickert  and  Hacker  prove  that  the 
transverse  division  is  accompHshed  during  the  formation  of  the 
second  polar  body. 

2.    Detailed  Evidence 

We  may  now  consider  some  of  the  evidence  in  detail,  though 
the  limits  of  this  work  will  only  allow  the  consideration  of  some 
of  the  best  known  cases.  We  may  first  examine  the  case  of  Ascaris, 
on  which  the  first  account  is  based.  In  the  first  of  his  classical 
cell-studies  Boveri  showed  that  each  tetrad  appears  in  the  ger- 
minal vesicle  in  the  form  of  four  parallel  rods,  each  consisting  of 
a  row  of  chromatin-granules  (Fig.  89,  A-C).  He  believed  these  rods 
to  arise  by  the  double  longitudinal  splitting  of  a  single  primary  chro- 
matin-rod,  each  cleavage  being  a  preparation  for  one  of  the  polar 
bodies.  In  his  opinion,  therefore,  the  formation  of  the  polar  bodies 
differs  from  ordinary  mitosis  only  in  the  fact  that  the  chromosomes 
split  very  early,  and  not  once,  but  twice,  in  preparation  for  two  rapidly 
succeeding  divisions  without  an  intervening  resting  period.  He  sup- 
ported this  view  by  further  observations  in  1890  on  the  polar  bodies 
of  Sagitta  and  several  gasteropods,  in  which  he  again  determined,  as 
he  believed,  that  the  tetrads  arose  by  double  longitudinal  splitting. 
An  essentially  similar  view  of  the  tetrads  was  taken  by  Hertwig  in 
1890,  in  the  spermatogenesis  of  Ascaris,  though  he  could  not  support 
this  conclusion  by  very  convincing  evidence.  In  1893,  finally,  Brauer 
made  a  most  thorough  and  apparently  exhaustive  study  of  their  origin 
in  the  spermatogenesis  of  Ascaris,  which  seemed  to  leave  no  doubt  of 
the  correctness  of  Bov-eri's  result.  Every  step  in  the  origin  of  the 
tetrads  from  the  reticulum  of  the  resting  spermatocytes  was  traced 
w^ith  the  most  painstaking  care.  The  first  step  observed  was  a  double 
splitting  of  the  chromatin-threads  in  the  reticulum,  caused  by  a  divi- 
sion of  the  chromatin-granules  into  four  parts  (Fig.  92,  A\  From 
the  reticulum  arises  a  continuous  spireme-thread,  which  from  its  first 
appearance  is  split  into  four  longitudinal  parts,  and  ultimately  breaks 
in  two  to  form  the  two  tetrads  characteristic  of  the  species.  These 
have  at  first  the  same  rod-like  form  as  those  of  the  germinal  vesicle. 
Later  they  shorten  to  form  compact  groups,  each  consisting  of  four 
spherical  chromosomes.  Brauer's  figures  are  very  convincing,  and, 
if  correct,  seem  to  leave  no  doubt  that  the  tetrads  here  arise  by  a 
double  longitudinal  splitting  of  the  spireme-thread,  initiated  even  in 
the  reticular  stage  before  a  connected  thread  has  been  formed.     If 


i88 


REDUCTION   OF   THE    CHROMOSOMES 


this  really  be  so,  there  can  be  here  no  reducing  division  in  Weis- 
mann's  sense.  The  reduction  of  chromatin,  caused  by  the  ensuing 
cell-division,  is  therefore  only  a  quantitative  mass-reduction,  as  Hert- 
wig  and  Brauer  insist,  not  a  qualitative  sundering  of  different  ele- 
ments, as  Weismann's  postulate  demands.^  The  work  of  Strasburger 
and  Guignard,  considered  at  p.  195,  has  given  in  principle  the  same 
general  result  in  the  flowering  plants,  though  the  details  of  the  pro- 
cess are  here  considerably  modified,  and  apparently  no  tetrads  are 
formed. 


D  E  F 

Fig.  93.  —  Origin  of  tlie  tetrads  by  ring-formation  in  the  spermatogenesis  of  the  mole-cricket 
Gryllotalpa.     [VOM  RATH.] 

A.  Primary  spermatocyte,  containing  six  double  rods,  each  of  which  represents  two  chromo- 
somes united  end  to  end  and  longitudinally  split  except  at  the  free  ends.  B.  C.  Opening  out  of 
the  double  rods  to  form  rings.  D.  Concentration  of  the  rings.  E.  The  rings  broken  up  into 
tetrads.    F.  First  division-figure  established. 

We  now  return  to  the  second  view,  referred  to  at  p.  186,  which 
accords  with  Weismann's  hypothesis,  and  flatly  contradicts  the  con- 
clusions drawn  from  the  study  of  Ascaris.  This  view  is  based  mainly 
on  the  study  of  arthropods,  especially  the  Crustacea  and  insects,  but 
has  been  confirmed  by  the  facts  observed  in  some  of  the  lower  verte- 
brata.  In  many  of  these  forms  the  tetrads  first  appear  in  the  form 
of  closed  rings,  each  of  which  finally  breaks  into  four  parts.  First 
observed  by  Henking  ('91)  in  the  insect  Pyrrochoris,  they  have  since 
been  found  in  other  insects  by  vom  Rath  and  Wilcox,  in  various  cope- 

^  In  an  earlier  paper  on  Branchipus  ('92)  Brauer  reached  an  essentially  similar  result, 
which  was,  however,  based  on  far  less  convincing  evidence. 


ORIGIN   OF  THE    TETRADS 


189 


pods  by  Ruckert,  Hacker,  and  vom  Rath,  in  the  frog  by  vom  Rath, 
and  in  elasmobranchs  by  Moore.  The  genesis  of  the  ring  was  first 
determined  by  vom  Rath  in  the  mole-cricket  {Gryllotalpa,  '92),  and 
has  been  thoroughly  elucidated  by  the  later  work  of  Riickert  ('94) 
and   Hackfer  ('95,  i).      All  these  observers,   excepting  Wilcox  and 


■■ooe 


'00^'o^s'o' 


D 


(The 


Fig.  94.  —  Formation  of  the  tetrads  and   polar  bodies  in   Cyclops,  slightly  schematic, 
full  number  of  tetrads  is  not  shown.)     [RiJCKERT.] 

A.  Germinal  vesicle  containing  eight  longitudinally  split  chromatin-rods  (half  the  somatic 
number).  B.  Shortening  of  tlie  rods;  transverse  division  (to  form  the  tetrads)  in  progress. 
C.  Position  of  the  tetr^ids  in  the  first  polar  spindle,  tlie  longitudinal  split  horizontal.  D.  Ana- 
phase ;  longitudinal  division  of  the  tetrads.  E.  The  first  polar  body  formed ;  second  polar 
spindle  with  the  eight  dyads  in  position  for  the  ensuing  division,  which  will  be  a  transverse  or 
reducing  division. 


Moore  (see  p.  201),  have  reached  the  same  conclusion;  namely,  that 
the  ring  arises  by  the  longitudinal  splitting  of  a  primary  chromatin- 
rod,  the  two  halves  remaining  united  by  their  ends,  and  opening  out 
to  form  a  ring.  The  ring-formation  is,  in  fact,  a  form  of  heterotypi- 
cal  mitosis  (p.  60).     The  breaking  of  the  ring  into  four  parts  involves 


190  REDUCTION  OF   THE    CHROMOSOMES 

first  the  separation  of  these  two  halves  (corresponding  with  the  origi- 
nal longitudinal  split),  and  second,  the  transverse  division  of  each  half, 
the  latter  being  the  reducing  division  of  Weismann.  The  number  of 
primary  rods,  from  which  the  rings  arise,  is  one-half  the  somatic 
number.  Hence  each  of  them  is  conceived  by  vom  Rath,  Hacker, 
and  Riickert  as  bivalent  or  double ;  i.e.  as  representing  two  chro- 
mosomes united  end  to  end.  This  appears  with  the  greatest  clear- 
ness in  the  spermatogenesis  of  Gryllotalpa  (Fig.  93).  Here  the 
spireme-thread  splits  lengthwise  before  its  segmentation  into  rods. 
It  then  divides  transversely  to  form  six  double  rods  (half  the  usual 
number  of  chromosomes),  which  open  out  to  form  six  closed  rings. 
These  become  small  and  thick,  break  each  into  four  parts,  and  thus 
give  rise  to  six  typical  tetrads.  An  essentially  similar  account  of  the 
ring-formation  is  given  by  vom  Rath  in  Euchceta  and  Calanus,  and 
by  Riickert  in  Heterocope  and  Diaptomus. 

That  the  foregoing  interpretation  of  the  rings  is  correct,  is  beauti- 
fully demonstrated  by  the  observations  of  Hacker,  and  especially  of 
Riickert,  on  a  number  of  other  copepods  {Cyclops,  Canthocanipttis\ 
in  which  rings  are  not  formed,  since  the  splitting  of  the  primary 
chromatin-rods  is  complete.  The  origin  of  the  tetrads  has  here  been 
traced  with  especial  care  in  Cyclops  strenims,  by  Riickert  ('94),  whose 
observations,  confirmed  by  Hacker,  are  quite  as  convincing  as  those 
of  Brauer  on  Ascaris,  though  they  lead  to  a  diametrically  opposite 
result. 

The  normal  number  of  chromosomes  is  here  twenty-two.  In  the 
germinal  vesicle  arise  eleven  threads,  which  split  lengthwise  (Fig.  94), 
and  finally  shorten  to  form  double  rods,  manifestly  equivalent  to  the 
closed  rings  of  Diaptomus.  Each  of  these  now  segments  trajisversely 
to  form  a  tetrad  group,  and  the  eleven  tetrads  then  place  themselves 
in  the  equator  of  the  spindle  for  the  first  polar  body  (Fig.  94,  C\  in  such 
a  manner  that  the  longitudinal  split  is  transverse  to  the  axis  of  the 
spindle.  As  the  polar  body  is  formed,  the  longitudinal  halves  of 
the  tetrad  separate,  and  the  formation  of  the  first  polar  body  is  thus 
demonstrated  to  be  an  "  equal  division  "  in  Weismann's  sense.  The 
eleven  dyads  remaining  in  the  eggs  now  rotate  (as  in  Ascaris),  so  that 
the  transverse  division  lies  in  the  equatorial  plane,  and  are  halved 
during  the  formation  of  the  second  polar  body.  The  division  is 
accordingly  a  "  reducing  division,"  which  leaves  eleven  single  chromo- 
somes in  the  Qgg,  and  it  is  a  curious  fact  that  this  conclusion,  which 
apparently  rests  on  irrefragable  evidence,  completely  confirms  Weis- 
mann's earlier  views,  published  in  1887,^  and  contradicts  the  later 
interpretation  upheld  in  his  book  on  the  germ-plasm. 

1  Essay  VI. 


ORTGTN  OF   THE    TETRADS 


191 


Hacker  ('92)  has  reached  exactly  similar  results  in  the  case  of 
CantJiocamptus  and  draws  the  same  conclusion.  In  Cyclops  stremuis 
he  finds  in  the  case  of  first-laid  eggs  a  variation  of  the  process  which 
seems  to  approach  the  mode  of  tetrad  formation  in  some  of  the  lower 
vertebrates'.'     In  such  eggs  the  primary  double  rods  become  sharply 


a 


Fig.  95.  —  Diagrams  of  various  modes  of  tetrad-formation.  [HacKER.] 
a.  Common  starting-point,  a  double  spireme-thread  in  the  germinal  vesicle ;  d.  common  re- 
sult, the  typical  tetrads;  b.  c.  intermediate  stages:  at  the  left  the  ring-formation  (as  in  Diaptomus, 
Gryllotalpa,  Heteiocope)  ;  middle  series,  complete  splitting  of  the  rods  (as  in  Cyclops  according  to 
Riickert,  and  in  Catifhocamptus)  ;  at  the  right  by  breaking  of  the  V-shaped  rods  (as  in  Cyclops 
strenuus,  according  to  Hacker,  and  in  the  salamander,  according  to  vom  Rath). 

bent  near  the  middle  to  form  V-shaped  loops  (Fig.  96,  C),  which  finally 
break  transversely  near  the  apex  to  form  the  tetrad  ^  —  a  process  which 
clearly  gives  the  same  result  as  before.  An  exactly  similar  process 
seems   to   occur  in  the  salamander  as   described   by  Flemming  and 

1  Hacker  upholds  this  account  ('95,  i)   in  spite  of  the  criticisms  of  Riickert  and  vom 

Rath. 


92 


REDUCTION  OF   THE    CHROMOSOMES 


vom  Rath.  Flemming  observed  the  double  V-shaped  loops  in  1887, 
and  also  the  tetrads  derived  from  them,  but  regarded  the  latter  as 
"anomalies."  Vom  Rath  ('93)  subsequently  found  that  the  double 
V's  break  at  the  apex,  and  that  the  four  rods  thus  formed  then  draw 
together  to  form  four  spheres  grouped  in  a  tetrad  precisely  like 
those  of  the  arthropods.  Still  later  ('95,  i)  the  same  observer  traced 
a  nearly  similar  process  in  the  frog ;  but  in  this  case  the  four  ele- 


Fig.  96.  —  Germinal  vesicles  of  various  eggs,  showing  chromosomes,  tetrads,  and  nucleoli. 

A.  A  copepod  {Hetei  ocope)  showing  eight  of  the  sixteen  ring-shaped  tetrads  and  the  nucleo- 
lus.    [RDCKERT.] 

B.  Later  stap-e  of  the  same,  condensation  and  segmentation  of  the  rings.     [ROCKERT.] 

C.  "Cyclops  stretiuwi"  illustrating  Hacker's  account  of  the  tetrad-formation  from  elongate 
double  rods  ;  a  group  of  "  accessory  nucleoli."     [HaCKF.r  ] 

D.  (terminal  vesicle  of  an  annelid  {Ophryotrocha)  showing  nucleolus  and  four  chromosomes. 
[KORSCHELT.] 

ments  appear  to  remain  for  a  short  time  united  to  form  a  ring  before 
breaking  up  into  separate  spheres. 

To  sum  up:  The  researches  of  Riickert,  Hacker,  and  vom  Rath, 
on  insects,  Crustacea,  and  amphibia  have  all  led  to  the  same  result. 
However  the  tetrad-formation  may  differ  in  matter  of  detail,  it  is  in 
all  these  forms  the  same  in  principle.  Each  primary  chromatin-rod 
has  the  value  of  a  bivalent  chromosome ;  i.e.  two  chromosomes 
joined  end  to  end,  ab.     By  a  longitudinal  division  a  ring  or  double 


THE  EARLY  HISTORY  OF   THE    GERM-NUCLEI  1 93 

rod  is  formed,  which  represents  two  equivalent  pairs  of  chromosomes 

—7-      During  the    two    maturation-divisions  the    four    chromosomes 

a\b 
are  spHt  apart,  — r7>    and    Riickert's  observations  demonstrate  that 

the  first  division  separates  the  two  equivalent  dyads,  ab  and  ab,  which 
by  the  second  division  are  split  apart  into  the  two  separate  chromo- 
somes, a  and  b.  Weismann's  postulate  is  accordingly  realized  in  the 
second  division.  It  is  clear  from  this  account  that  the  primary 
halving  of  the  number  of  chromatin-rods  is  not  an  actual  reduction, 
since  each  rod  represents  two  chromosomes.  Riickert  therefore 
proposes  the  convenient  term  ''  pseudo-reduction "  for  this  pre- 
liminary halving.^  The  actual  reduction  is  not  effected  until  the 
dyads  are  split  apart  during  rhe  second  maturation-division. 


C.     The  Early  History  of  the  Germ-Nuclei 

We  may  for  the  present  defer  a  consideration  of  accounts  of  reduc- 
tion differing  from  the  two  already  described  and  pass  on  to  a 
consideration  of  the  earlier  history  of  the  germ-nuclei.  A  consider- 
able number  of  observers  are  now  agreed  that  the  primary  chromatin- 
rods  appear  at  a  very  early  period  in  the  germinal  vesicle  and  are 
longitudinally  split  from  the  first.  (Hacker,  vom  Rath,  Riickert,  in 
copepods ;  Riickert  in  selachians ;  Born  and  Fick  in  amphibia ; 
Holl  in  the  chick  ;  Riickert  in  the  rabbit.)  Hacker  ('92,  2)  made  the 
interesting  discovery  that  in  some  of  the  copepods  (yCanthocamptiis^ 
Cyclops)  these  double  rods  could  be  traced  back  continuously  to  a 
double  spireme-thread,  following  immediately  upon  the  division  of  the 
last  generation  of  oogonia,  and  that  at  no  peinod  is  a  true  reticiihnn 
formed  in  tJie  germinal  vesicle  (Fig.  97).  In  the  following  year  Riick- 
ert ('93,  2)  made  a  precisely  similar  discovery  in  the  case  of  selachians. 
After  division  of  the  last  generation  of  oogonia  the  daughter-chro- 
mosomes do  not  give  rise  to  a  reticulum,  but  split  lengthwise,  and 
persist  in  this  condition  throughout  the  entire  growth-period  of  the 
^^g.  Riickert  therefore  concluded  that  the  germinal  vesicle  of  the 
selachians  is  to  be  regarded  as  a  "  daughter-spireme  of  the  oogonium 
(Ur-ei)  grown  to  enormous  dimensions,  the  chromosomes  of  which 
are  doubled  and  arranged  in  pairs."  ^  In  the  following  year  ('93) 
vom  Rath,  following  out  the  earlier  work  of  Flemming,  discovered 
an  exactly  analogous  fact  in  the  spermatogenesis  of  the  salamander. 
The  tetrads  were  here  traced  back  to  double  chromatin-rods,  indi- 
vidually identical  with  the  daughter-chromosomes  of  the  preceding 


194 


REDUCTION  OF   THE    CHROMOSOMES 


spermatogonium-division,  which  spHt  lengthwise  during  the  anaphase 
and  pass  into  the  spermatocyte-nucleus  without  forming  a  reticulum. 
Flemming  had  observed  in  1887  that  these  daughter-chromosomes 
split  in  the  anaphase,  but  could  not  determine  their  further  history. 
Vom  Rath  found  that  each  double  daughter-chromosome  breaks  in 
two  at  the  apex  to  form  a  tetrad,  which  passes  into  the  ensuing 
spermatocyte  without  the  intervention  of  a  resting  stage. ^ 

It  is  clear  that  in  such  cases  the  "  pseudo-reduction  "  must  take 
place  at  an  earlier  period  than  the  penultimate  generation  of  cells. 
In  the  salamander  Flemming  i'^j)  found  that  the  "•  chromosomes  "  of 
the  spermatogonia  appeared  in  the  reduced  number  (twelve)  in  at  least 
three  cell-generations  preceding  the  penultimate.     Vom   Rath  ('93) 


Fig.  97.  —  Longitudinal  section  through  the  ovary  of  the  copepod  Canthocatnptus.     [HACKER.] 
og.  The   youngest  germ-cells  or  oogonia  (dividing  at  og.'^  ;    a.  upper  part  of  the  growth- 
zone  ;  oc.  oocyte,  or  growing  ovarian  egg ;  ov.  fully  formed  t,^,'g,  with  double  chromatin-rods. 


traced  the  pseudo-reduction  in  both  sexes  back  to  much  earlier  stages, 
not  only  in  the  larvae,  but  even  in  the  embryo  (!).  This  very  remark- 
able discovery  showed  that  tJie  pseudo-reductio7i  might  appear  in  the 
early  progenitors  of  the  germ-cells  during  embryonie  life  — perhaps  even 
during  the  cleavage.  This  conjecture  has  apparently  been  substan- 
tiated by  Hacker  ('95,  3),  who  finds  that  in   Cyclops  brevicornis  the 

1  It  is  certain  that  these  facts  do  not  represent  a  universal  type  of  maturation,  for  in 
Ascaris  there  is  no  doubt  that  a  true  reticular  resting  stage  occurs  in  the  primary  spermato- 
cytes, and  probably  also  in  the  germinal  vesicle.  Hacker  found,  moreover,  that  the  same 
species  might  show  differences  in  this  regard;  for  in  Cyclops  strenitus  the  first-laid  eggs  have 
no  resting  stage,  the  double  daughter-chromosome  passing  directly  into  the  tetrads,  while 
in  later  broods  of  eggs  a  daughter-spireme,  composed  of  long  double  threads,  is  formed.  The 
difference  is  believed  by  Hacker  to  be  due  to  the  fact  that  the  earlier  eggs  are  quickly  laid, 
while  the  later  broods  arc  long  retained  in  the  oviduct. 


REDUCTION  IN   THE   PLANTS  1 95 

reduced  number  of  chromosomes  (twelve)  appears  in  the  primordial 
germ-cells  which  are  differentiated  in  the  blastula-stage  (Fig.  56). 
He  adds  the  interesting  discovery  that  in  this  form  the  somatic  nuclei 
of  the  cl^vage-stages  show  the  same  number,  and  hence  concludes 
that  all  the  chromosomes  of  these  stages  are  bivalent.  As  develop- 
ment proceeds,  the  germ-cells  retain  this  character,  while  the  somatic 
cells  acquire  the  usual  number  (twenty-four) —  a  process  which,  if  the 
conception  of  bivalent  chromosomes  be  valid,  must  consist  in  the 
division  of  each  bivalent  rod  into  its  two  elements.  We  have  here  a 
wholly  new  light  on  the  historical  origin  of  reduction  ;  for  the  pseudo- 
reduction  of  the  germ-nuclei  seems  to  be  in  this  case  a  persistence 
of  the  embryonic  condition,  and  we  may  therefore  hope  for  a  future 
explanation  of  the  process  by  which  it  has  in  other  cases  been 
deferred  until  the  penultimate  cell-generation,  as  is  certainly  the  fact 
in  Ascai'is}  The  foregoing  facts  pave  the  way  to  an  examination  of 
reduction  in  the  plants,  to  which  we  now  proceed. 

D.     Reduction  in  the  Plants 

Guignard's  and  Strasburger's  observations  on  reduction  in  the 
flowering  plants  gave  a  result  which  in  substance  agrees  with  that 
obtained  by  Boveri  and  Brauer  in  the  case  of  Ascaris.  These 
observers  could  find  absolutely  no  evidence  of  a  transverse  or  reduc- 
ing division,  and  asserted  that  the  reduction  in  number  is  directly 
effected  by  a  segmentation  of  the  spireme-thread  into  half  the  usual 
number  of  chromosomes  ;  i.e.  by  a  process  exactly  corresponding 
with  the  "pseudo-reduction"  of  Ruckert  (see  Fig.  25).  These 
observers  find  that  in  the  male  the  chromosomes  suddenly  appear 
in  the  reduced  number  (twelve  in  the  lily,  eight  in  the  onion)  at 
the  first  division  of  the  pollen-mother-cell,  from  which  arise  four 
pollen-grains.  In  the.  female  the  same  process  takes  place  at  the 
first  division  of  the  mother-cell  of  the  embryo-sac.  Strasburger 
and  Guignard  agree  that  in  the  subsequent  divisions  these  chromo- 
somes do  not  form  tetrads,  but  tmdergo  simple  longitudinal  split- 
ting at  each  successive  division.  In  case  of  the  male  there  are 
at  least  four  of  these  divisions ;  viz.  two  divisions  to  form  the 
four  pollen-grains,  a  third  division  to  form  the  vegetative  and 
generative  cell  of  the  pollen-grain,  and  finally  a  fourth  division 
of  the  generative  nucleus  in  the  pollen-tube.  In  all  these  mitoses 
the  reduced  number  of  chromosomes  appears,  and  each  division  is 
followed  by  a   return  of  the   nucleus   to  the  resting  state.      In  the 

^  It  may  be  recalled  that  in  Ascaris  Boveri  proved  that  the  primordial  germ-cells  have  the 
full  number  of  chromosomes,  and  Hertwig  clearly  showed  that  this  number  is  retained  up 
to  the  last  division  of  the  spermatogonia. 


196  REDUCTION  OF   THE    CHROMOSOMES 

mother-cell  of  the  embryo-sac  the  number  of  divisions  before  fertiliza- 
tion is  three,  four,  five,  or  sometimes  even  more,  the  reduced  number 
persisting  throughout.  These  facts  led  to  the  suspicion,  first  expressed 
by  Overton  in  1892,  that  the  reduced  number  of  chromosomes  might  be 
found  in  the  sexual  generation  of  higher  cryptogams  (which  corresponds 
with  the  cells  derived  from  the  pollen-grain,  or  from  the  mother-cell  of 
the  embryo-sac).  This  surmis'e  quickly  became  a  certainty.  Overton 
himself  discovered  ('93)  that  the  cells  of  the  endosperm  in  the 
Gymnosperm  Ceratozamia  divide  with  the  reduced  number,  namely 
eight ;  and  Dixon  observed  the  same  fact  in  Pinits  at  the  same  time. 
In  the  following  year  Strasburger  brought  the  matter  to  a  definite 
conclusion  in  the  case  of  a  fern  {Osmiinda),  showing  that  all  the  cells 
of  the  protJiallium,  from  the  original  spore-motJier-cell  onivards  to  the 
formation  of  the  germ-cells,  have  one-half  the  number  of  chromosomes 
found  in  the  asexual generatioji,  namely  twelve  instead  of  twenty-four; 
in  other  words,  the  reduction  takes  place  in  the  formation  of  the  spore 
from  which  the  sexual  generation  arises,  scores  of  cell-generations 
before  the  germ-cells  are  formed,  indeed  before  the  formation  of  the 
body  from  which  these  cells  arise.  Similar  facts  were  determined  by 
Farmer  in  Pallavicinia,  one  of  the  Hepaticae,  where  all  of  the  nuclei 
of  the  asexual  generation  (sporogonium)  show  four  chromosomes  dur- 
ing division,  those  of  the  sexual  generation  (thallus)  eight.  It  now 
seems  highly  probable  that  this  will  be  found  a  general  rule. 

The  striking  point  in  these,  as  in  vom  Rath's  and  Hacker's  obser- 
vations, is  that  the  numerical  reduction  takes  place  so  long  before 
the  fertilization  for  which  it  is  the  obvious  preparation.  Speculating 
on  the  meaning  of  this  remarkable  fact,  Strasburger  advances  the 
hypothesis  that  the  reduced  number  is  the  ancestral  number  inherited 
from  the  ancestral  type.  The  normal,  i.e.  somatic,  number  arose 
through  conjugation  by  which  the  chromosomes  of  two  germ-cells 
were  brought  together.  Strasburger  does  not  hesitate  to  apply  the 
same  conception  to  animals,  and  suggests  that  the  four  cells  arising  by 
the  division  of  the  oogonium  {Qgg  plus  three  polar-bodies)  represent 
the  remains  of  a  separate  generation,  now  a  mere  remnant  included 
in  the  body  in  somewhat  the  same  manner  that  the  rudimentary  pro- 
thallium  of  angiosperms  is  included  in  the  embryo-sac.  This  may 
seem  a  highly  improbable  conclusion,  but  it  must  not  be  forgotten 
that  so  able  a  zoologist  as  Whitman  expressed  a  nearly  related 
thought,  as  long  ago  as  1878:  **  I  interpret  the  formation  of  polar 
globules  as  a  relic  of  the  priinitive  mode  of  asexual  reproduction.''^ 
Could  Strasburger's  hypothesis  be  substantiated,  it  would  place  the 
entire  problem,  not  merely  of  maturation,  but  of  sexuality  itself,  in 
a  new  light. 

1  '78,  p.  262. 


REDUCTION  IN   THE   PLANTS 


[97 


Strasburger's  hypothesis  is,  however,  open  to  a  very  serious  a 
priori  objection,  as  Hacker  has  pointed  out ;  for  if  the  account  of 
"  reduction  "  in  the  plants  given  by  Guignard  and  Strasburger  be 
correct,  it^corresponds  exactly  to  the  "  pseudo-reduction  "  in  animals, 
and  the  "  chromosomes  "  of  the  sexual  generation  must  be  bivalent 
like  those  of  the  early  germ-cells  in  animals.  The  recent  observa- 
tions of  Belajeff,  Farmer,  and  especially  those  of  Sargant,  give,  how- 
ever, good  reason  to  believe  that  both  Guignard  and  Strasburger  have 
overlooked  some  of  the  most  essential  phenomena  of  reduction. 
These  observations  have  not  yet  revealed  the  exact  nature  of  the 
process,  yet  they  show  that 
the  first  division  of  the  pollen- 
mother-cells  (in  the  lily)  is  of 
the  Jicterotypical  form  ;  i.e. 
that  the  cJirornosomes  have  the 
form  of  rings.  It  is  impos- 
sible to  avoid  the  suspicion 
that  these  rings  may  be  of 
the  same  nature  as  the  ring- 
shaped  tetrads  in  animals, 
though  apparently  they  do 
not  actually  break  up  into 
a  tetrad.  Until  this  point 
has  been  cleared  up  by  fur- 
ther investigation  the  nature 
of  reduction  in  the  plants 
remains  an  open  question. 
Belajeff  and  Farmer  showed 
that  as  the  daughter-chromo- 
somes diverge  after  the  first 
division  they  assume  a  V- 
shape,  and  Miss  Sargant's 
very  interesting  observations 
give  some  reason  to  believe 
that  the  V  breaks  at  the  apex  precisely  as  described  by  Hacker  in 
Cyclops  and  vom  Rath  in  the  salamander  (Fig.  98,  g).  Should  this 
prove  to  be  the  case  the  way  would  be  opened  for  an  interpretation 
of  reduction  in  the  plants  agreeing  in  principle  with  that  of  Riick- 
ert,  Hacker,  and  vom  Rath ;  and  as  far  as  the  plants  are  concerned, 
the  a  priori  objection  to  Strasburger's  interesting  hypothesis  might 
be  removed. 


Fig.  98.  —  Division  of  the  chromosomes  (?  tetrad- 
formation)  in  the  first  division  of  the  pollen-mother- 
cell  of  the  lily.  {a.b.  after  FARMER  and  MoORE; 
c-g.  after  Sargant.) 

a.  b.  Two  stages  in  the  ring-formation  (hetero- 
typical  mitosis),  c-f.  Successive  stages,  in  profile 
view,  of  the  separation  of  the  daughter-chromosomes. 
g.  The  daughter-chromosomes,  seen  en  face,  at  the 
moment  of  separation ;  this  stage  is  perhaps  to  be 
interpreted  as  a  tetrad  like  those  occurring  in  the 
salamander. 


98 


REDUCTION  OF   THE    CHROMOSOMES 


E.     Reduction  in  Unicellular  Forms 

A  reduction  of  the  number  of  chromosomes  as  a  preparation  for 
conjugation  in  the  one-celled  forms  has  not  yet  been  certainly  deter- 
mined, but  there  are  many  facts  that  render  it  highly  probable.     In 


C 


D 


Fig- 99-— Conjugation  of  C7oj/<fr/«;;/.  [Klebahn.] 
A.  Soon  after  union,  four  chromalophores.  B.  Chromafophores  reduced  totwo,  rruclei 
distinct,  C.  Fusion  of  the  nuclei.  D.  First  cleavage  of  the  zygote.  E.  Rfsulting  2-cell  stage. 
E.  Second  cleavage.  G.  Resulting  stage,  each  cell  bi-nucleate.  •  //.  Separation  of  the  cells ; 
one  of  the  nuclei  in  each  enlarging  to  form  the  permanent  nucleus,  the  other  (probably  repre- 
senting a  polar  body)  degenerating. 


DIVERGENT  ACCOUNTS   OF  REDUCTION  1 99 

the  conjugation  of  infusoria,  as  already  described  (p.  165),  the  original 
nucleus  divides  several  times  before  union,  and  only  one  of  the  result- 
ing nuclei  becomes  the  conjugating  germ-nucleus,  while  the  others 
perish,  like  the  polar  bodies.  The  numerical  correspondence  be- 
tween the  rejected  nuclei  or  "corpuscles  de  rebut"  has  already  been 
pointed  out  (p.  168).  Hertwig  could  not  count  the  chromosomes 
with  absolute  certainty,  yet  he  states  ('89)  that  in  Paramoechnn 
caitdatum,  during  the  final  division,  the  number  of  spindle-fibres 
and  of  the  corresponding  chromatic  elements  is  but  4-6,  while  in 
the  earlier  divisions  the  number  is  approximately  double  this  (8-9). 
This  observation  makes  it  nearly  certain  that  a  numerical  reduction 
of  chromosomes  occurs  in  the  Protozoa  in  a  manner  similar  to  that 
of  the  higher  forms ;  but  the  reduction  here  appears  to  be  deferred 
until  the  final  division.^  In  the  gregarines  Wolters('9i)has  observed 
the  formation  of  an  actual  polar  body  as  a  small  cell  segmented  off 
from  each  of  the  two  conjugating  animals  soon  after  their  union ; 
but  the  number  of  chromosomes  was  not  determined. 

In  the  unicellular  plants  there  are  indications  of  a  similar  process, 
but  the  few  facts  at  our  command  indicate  that  the  reduction  may 
here  take  place  not  before,  but  after-,  conjugation  of  the  nuclei.  Thus 
in  the  dermids  Closteruim  and  Cosrnariuin,  according  to  Klebahn 
(Fig.  99),  the  nuclei  first  unite  to  form  a  cleavage-nucleus,  after  which 
the  zygote  divides  into  two.  Each  of  the  new  nuclei  now  divides, 
one  of  the  products  persisting  as  the  permanent  nucleus,  while  the 
other  degenerates  and  disappears.  Chmielewski  asserts  that  a  similar 
process  occurs  in  Spirogyra.  Although  the  numerical  relations  of 
the  chromosomes  have  not  been  determined  in  these  cases,  it  appears 
probable  that  the  elimination  of  a  nucleus  in  each  cell  is  a  process  of 
reduction  occurring:  after  fertilization. 


F.     Divergent  Accounts  of  Reduction 

We  can  only  touch  on  a  few  of  the  accounts  of  reduction  which 
differ  from  both  the  modes  already  considered.  Of  these  the  most 
interesting  are  observations  which  indicate  the  possibility  of, 

I.    TJie  Foiination  of  Tetrads  by  Conjugation 

A  considerable  number  of  observers  have  maintained  that  reduc- 
tion may  be  effected  by  the  union  or  conjugation  of  chromosomes 
that  were  previously  separate.  This  view  agrees  in  principle  with 
that  of  Riickert,  Hacker,  and  vom  Rath ;  for  the  bivalent  chromo- 

^  Cf.  Moore  on  the  spermatogenesis  of  mammals,  p.  201. 


200  REDUCl^ION  OF   THE    CHROMOSOMES 

somes  assumed  by  these  authors  may  be  conceived  as  two  conjugated 
chromosomes.  It  seems  to  be  confirmed  by  the  observations  of  Born 
and  Fick  on  amphibia  and  those  of  Riickert  on  selachians  {Pristi- 
iirus) ;  for  in  all  these  cases  the  number  of  chromatin-masses  at  the 
time  the  first  polar  body  is  formed  is  but  half  the  number  observed 
in  younger  stages  of  the  germinal  vesicle.  In  Pristiurus  there  are 
at  first  thirty-six  double  segments  in  the  germinal  vesicle.  At  a  later 
period  these  give  rise  to  a  close  spireme,  which  then  becomes  more 
open,  and  is  found  to  form  a  double  thread  segmented  into  eighteen 
double  segments ;  i.e.  the  reduced  number.  In  this  case,  therefore, 
the  preliminary  pseudo-reduction  is  almost  certainly  effected  by  the 
union  of  the  original  thirty-six  double  chromosomes,  two  by  two. 
The  most  specific  accounts  of  such  a  mode  of  origin  have,  however, 
been  given  by  Calkins  (earthworm)  and  Wilcox  (grasshopper).  The 
latter  author  asserts  ('95)  that  in  Caloptemis  the  spireme  of  the  first 
spermatocyte  first  segments  into  the  normal  number  (twelve)  of  dumb- 
bell-shaped segments,  which  then  become  associated  in  pairs  to  form 
six  tetrads.  Each  of  these  dumb-bell-shaped  bodies  is  assumed  to  be 
a  bivalent  chromosome,  and  the  tetrad-formation  is  therefore  inter- 
preted as  follows :  — 
abcd-l  ab-cd-kl  ^  a\b_  e\f_^    etc.  (tetrads). 

(spireme)     '  (segmented  spireme)  c\d  fC\^^ 

There  is,  therefore,  no  longitudinal  splitting  of  the  chromosomes. 
A  careful  examination  of  the  figures  does  not  convince  me  of  the 
correctness  of  this  conclusion,  which  is,  moreover,  inconsistent  with 
itself  on  Wilcox's  own  interpretation.  Since  each  germ-nucleus 
receives  six  chromosomes,  the  somatic  number  must  be  12,  and 
Wilcox  has  observed  this  number  in  the  divisions  of  the  sperma- 
togonia. The  12  dumb-bell-shaped  primary  segments  must  there- 
fore   represent   single   chromosomes,  not   bivalent   ones,  as  Wilcox 

assumes,  and  his  primary  tetrad  must  therefore  be  not  — L^>  as  he 

assumes,  but  either  -  or  (if  we  assume  that  the  normal  number  of 

I 
chromosomes  undergoes  a  preliminary  doubling)  -rv-r'       Until  this 

contradiction  is  cleared  up  Wilcox's  results  must  be  received  with 
considerable  scepticism. 

The  second  case,  which  is  perhaps  better  founded,  is  that  of  the 
earthworm  {L?imbricus  tcii-estris),  as  described  by  Calkins  ('95,  2), 
whose  work  was  done  under  my  own  direction.  Calkins  finds,  in 
accordance  with  all  other  spermatologists  save  Wilcox,  that  the 
spireme-thread  splits  longitudinally  and  then  divides  transversely 
into  32  double  segments.  These  then  unite,  two  by  two,  to  form 
16  tetrads.     The   32   primary  double  segments  therefore  represent 


DIVERGENT  ACCOUNTS   OF  REDUCTION  20I 

chromosomes  of  the  normal  number  that  have  spHt  longitudinally, 

b 


i.e. T'  etc.,  and  the  formula  for  a  tetrad  is  — 

a      b  a 


J  °' 


Such 


a  tetrad,  therefore,  agrees  as  to  its  composition  with  the  formulas  of 
Hacker,  vom  Rath,  and  Riickert,  and  agrees  in  mode  of  origin  with 
the  process  described  by  Riickert  in  the  eggs  of  Pristitiriis.  While 
these  observations  are  not  absolutely  conclusive,  they  nevertheless 
rest  on  strong  evidence,  and  they  do  not  stand  in  actual  contradiction 
of  what  is  known  in  the  copepods  and  vertebrates.  The  possibility 
of  such  a  mode  of  origin  in  other  forms  must,  I  think,  be  held  open. 

Under  the  same  category  must  be  placed  Korschelt's  unique 
results  in  the  egg-reduction  of  the  annelid  Ophryotrocha  ('95),  which 
are  very  difficult  to  reconcile  with  anything  known  in  other  forms. 
The  typical  somatic  number  of  chromosomes  is  here  four.  The  same 
number  of  chromosomes  appear  in  the  germinal  vesicle  (Fig.  96,  D). 
They  are  at  first  single,  then  double  by  a  longitudinal  split,  but  after- 
wards single  again  by  a  reunion  of  the  halves.  The  four  chromo- 
somes group  themselves  in  a  single  tetrad,  two  passing  into  the  first 
polar-body,  while  two  remain  in  the  ^^^,  but  meanwhile  each  of  them 
again  splits  into  two.  Of  the  four  chromosomes  thus  left  in  the  ^g% 
two  are  passed  out  into  the  second  polar  body,  while  the  two  remain- 
ing in  the  Q,gg  give  rise  to  the  germ-nucleus.  From  this  it  follows 
that  the  formation  of  the  first  polar  body  is  a  reducing  division  (!) 
—  a  result  which  agrees  with  the  earlier  conclusions  of  Henking  on 
PyrrocJioris,  but  differs  entirely  from  those  of  Riickert,  Hacker,  and 
vom  Rath.  The  meaning  of  this  remarkable  result  cannot  here  be 
discussed.  A  clue  to  its  interpretation  is  perhaps  given  by  Hacker's 
interesting  observations  on  the  two  modes  of  maturation  in  Cantho- 
camptus,  for  which  the  reader  is  referred  to  Hacker's  paper  ('95,  i). 

Moore  ('95)  has  given  an  account  of  reduction  in  the  spermatogen- 
esis of  mammals  and  elasmobranchs  which  differs  widely  in  many 
respects  from  those  of  all  other  observers.  In  both  cases  there  is 
said  to  be  a  resting  stage  between  the  two  spermatocyte-divisions, 
and  in  mammals  (rat)  the  reduced  number  of  chromosomes  first 
appears  in  the  prophase  of  the  last  division.  In  elasmobranchs  both 
spermatocyte-divisions  are  of  the  heterotypical  form,  with  ring- 
shaped  chromosomes.  On  all  these  points  Moore's  account  contra- 
dicts those  of  all  other  investigators  of  reduction  in  the  animals, 
and  he  is  further  in  contradiction  with  Riickert  on  the  number 
of  chromosomes.  His  general  interpretation  accords  with  that  of 
Brauer  and  Strasburger,  reducing  divisions  being  totally  denied. 
The  evidence  on  which  this  interpretation  rests  will  be  found  in  his 
original  papers. 


202  REDUCTION  OF   THE    CHROMOSOMES 


G.     Maturation  of  Parthenogenetic  Eggs 

The  maturation  of  eggs  that  develop  without  fertihzation  is  a  sub- 
ject of  special  interest,  partly  because  of  its  bearing  on  the  general 
theory  of  fertilization,  partly  because  it  is  here,  as  I  believe,  that  one 
of  the  strongest  supports  is  found  for  the  hypothesis  of  the  individ- 
uality of  chromosomes.  In  an  early  article  by  Minot  i^jj)  on  the 
theoretical  meaning  of  maturation  the  suggestion  is  made  that 
parthenogenesis  may  be  due  to  failure  on  the  part  of  the  ^g^  to 
form  the  polar  bodies,  the  egg-nucleus  thus  remaining  hermaphrodite, 
and  hence  capable  of  development  without  fertilization.  This  sug- 
gestion forms  the  germ  of  all  later  theories  of  parthenogenesis.  Bal- 
four {'80)  suggested  that  the  function  of  forming  polar  cells  has  been 
acquired  by  the  ovum  for  the  express  purpose  of  preventing  parthe- 
nogenesis, and  a  nearly  similar  view  was  afterwards  maintained  by 
Van  Beneden.^  These  authors  assumed  accordingly  that  in  par- 
thenogenetic eggs  no  polar  bodies  are  formed.  Weismann  i^?)G) 
soon  discovered,  however,  that  the  parthenogenetic  eggs  of  Poly- 
pJiemus  (one  of  the  Daphnidae)  produce  a  single  polar-body.  This 
observation  was  quickly  followed  by  the  still  more  significant  dis- 
covery by  Blochmann  i^Z'^)  that  in  Aphis  the  parthenogenetic  eggs 
produce  a  single  polar  body  while  the  fertilised  eggs  produce  two. 
Weismann  was  able  to  determine  the  same  fact  in  ostracodes  and 
rotif era,  and  was  thus  led  to  the  view  ^  which  later  researches  have 
entirely  confirmed,  that  it  is  the  second  polar  body  that  is  of  special 
significance  in  parthenogenesis.  Blochmann  observed  that  in  insects 
the  polar  bodies  were  not  actually  thrown  out  of  the  ^gg,  but 
remained  embedded  in  its  substance  near  the  periphery.  At  the 
same  time  Boveri  i^Zy,  i)  discovered  that  in  Ascaris  the  second  polar 
body  might  in  exceptional  cases  remain  in  the  ^gg  and  there  give 
rise  to  a  resting-nucleus  indistinguishable  from  the  egg-nucleus  or 
sperm-nucleus.  He  was  thus  led  to  the  interesting  suggestion  that 
parthenogenesis  might  be  due  to  the  retention  of  the  second  polar 
body  in  the  ^gg  and  its  union  with  the  egg-nucleus.  **  The  second 
polar  body  would  thus,  in  a  certain  sense,  assume  the  role  of  the 
spermatozoon,  and  it  might  not  without  reason  be  said  :  PartJieno- 
genesis  is  the  result  of  fertilization  by  the  second  polar  body ''  ^ 

This  conclusion  received  a  brilliant  confirmation  through  the  obser- 
vations of  Brauer  ('93)  on  the  parthenogenetic  ^gg  of  Ai'ternia, 
though  it  appeared  that  Boveri  arrived  at  only  a  part  of  the  truth. 
Blochmann  ('88-89)  had  found  that  in  the  parthenogenetic  eggs  of 
the  honey-bee,  tzvo  polar-bodies  are  formed,  and  Platner  discovered  the 

'  '83.  p.  622.  2  Essay  VI.,  p.  359.  3  i,-^^  p_  y^. 


MATURATION  OF  PARTHENOGENETIC  EGGS 


203 


same  fact  in  the  butterfly  Liparis  ('89)  — a  fact  which  seemed  to  con- 
tradict Boveri's  hypothesis.  Brauer's  beautiful  researches  resolved 
the  contradiction  by  showing  that  there  are  two  types  oi  parthenogene- 
sis which  rrjay  occur  in  the  same  animal.     In  the  one  case  Boveri's 


-oo^^^M^ 


0-cyoboU 


1:. 


Fig.  100.  —  First  type  of  maturation  in  the  parthenogenetic  egg  of  Artemia.  [BrAUER.] 
A.  The  first  polar  spindle;  the  equatorial  plate  contains  84  tetrads.  B,  C.  Formation  of  the 
first  polar  body ;  84  dyads  remain  in  the  egg  and  these  give  rise  to  the  egg-nucleus,  shown  in  D. 
F.  Appearance  of  the  egg-centrosqme  and  aster.  E.  G.  Division  of  the  aster  and  formation 
of  the  cleavage-figure ;  the  equatorial  plate  consists  of  84  apparently  single  but  in  reality  bivalent 
chromosomes. 


conception  is  exactly  realized,  while  the  other  is  easily  brought  into 
relation  with  it. 

{a)  In  both  modes  typical  tetrads  are  formed  in  the  germ-nucleus 
to  the  number  of  eighty-four.  In  the  first  and  more  frequent  case 
(Fig.  100)  but  one  polar  body  is  formed,  which  removes  eighty-four 
dyads,  leaving  eighty-four  in  the  Qgg.  There  may  be  an  abortive 
attempt  to  form  a  second  polar  spindle,  but  no  division  results,  and 


204 


REDUCTION   OF   THE    CHROMOSOMES 


the  eighty -four  dyads  give  rise  to  a  reticular  cleavage-nucleus.  From 
this  arise  eighty-four  thread-like  chromosomes,  and  the  same  numbei- 
appears  ifi  later  cleavage-stages. 

{b)  It  is  the  second  and  rarer  mode  that  realizes  Boveri's  concep- 
tion (Fig.  loi).  Both  polar  bodies  are  formed,  the  first  removing 
eighty-four  dyads  and  leaving  the  same  number  in  the  ^g'g.     In  the 


CO,',     "-,     ^^" 


•.V;.  -^ 


D  E 

Fig.  lOi.  —  Second  type  of  maturation  in  the  parthenogenetic  egg  of  Artemia.     [Brauer.] 
A.  Formation  of  second  polar  body.     D.  Return  of  the  second  polar  nucleus  {p.  b!^)  into  the 
^g'g;  development   of  the  egg-am phiaster.     C.  Union  of  the  egg-nucleus   (9)   with   the  second 
polar  nucleus    {p.b.'^).      D.  Cleavage-nucleus  and  amphiaster.      E.   First   cleavage-figure  with 
equatorial  plate  containing  i68  chromosomes  in  two  groups  of  84  each. 


formation  of  the  second,  the  eighty-four  dyads  are  halved  to  form 
two  daughter-groups,  each  containing  eighty-four  single  chromosomes. 
Both  these  groups  reniam  in  the  egg,  and  each  gives  rise  to  a  single 
reticular  nucleus,  as  described  by  Boveri  in  Ascaris.  These  tzvo  fiuclei 
place  themselves  side  by  side  in  the  cleavage  figure,  and  give  rise  each 
to  eighty-four  chromosomes,  precisely  like  tzvo  germ-nuclei  in  ordinary 
fertilisatiofi.     The  one  hundred  and  sixty-eight  chromosomes  split 


i 


SUMMARY  AND    CONCLUSION  20$ 

lengthwise,  and  are  distributed  in  the  usual  manner,  and  reappear 
in  the  same  number  in  all  later  stages.  In  other  words,  the  second 
polar  body  here  plays  the  part  of  a  sperm-nucleus,  precisely  as  main- 
tained by  Kk)veri. 

In  all  individuals  arising  from  eggs  of  the  first  type,  therefore,  the 
somatic  number  of  chromosomes  is  eighty-four ;  in  all  those  arising 
from  eggs  of  the  second  type,  it  is  one  hundred  and  sixty-eight.  It 
is  impossible  to  doubt  that  the  chromosomes  of  the  first  class  are 
bivalent;  i.e.  represent  two  chromosomes  joined  together  —  for  that 
the  dyads  have  this  value  is  not  a  theory,  but  a  known  fact.  It 
remains  to  be  seen  whether  these  facts  apply  to  other  parthenogenetic 
eggs ;  but  the  single  case  of  Arteinia  is  little  short  of  a  demonstration 
not  only  of  Hacker's  and  vom  Rath's  conception  of  bivalent  chromo- 
somes, but  also  of  the  more  general  hypothesis  of  the  individuality 
of  chromosomes  (Chapter  VI.).  Only  on  this  hypothesis  can  we 
explain  the  persistence  of  the  original  number  of  chromosomes, 
whether  eighty-four  or  one  hundred  and  sixty-eight,  in  the  later  stages. 
How  important  a  bearing  this  case  has  on  Strasburger's  theory  of 
reduction  (p.  196)  is  obvious. 


H.     Summary  and  Conclusion 

The  one  fact  of  maturation  that  stands  out  with  perfect  clearness 
and  certainty  amid  all  the  controversies  surrounding  it  is  a  reduction 
in  the  niunber  of  chromosomes  in  the  ultimate  germ-cells  to  one-half  the 
number  characteristic  of  the  somatic  cells.  It  is  equally  clear  that  this 
reduction  is  a  preparation  of  the  germ-cells  for  their  subsequent  union, 
and  a  means  by  which  the  number  of  chromosomes  is  held  constant 
in  the  species.  As  soon,  however,  as  we  attempt  to  advance  beyond 
this  point  we  enter  upon  doubtful  ground,  which  becomes  more  and 
more  uncertain  as  we  proceed.  With  a  few  exceptions  the  reduction 
in  number  first  appears  in  the  direct  progenitors  of  the  germ-cells  by 
a  segmentation  of  the  spireme-tJiread  into  onc-haf  tJie  nsual  number  of 
rods.  This  process  is,  however,  not  an  actual  reduction  in  the  num- 
ber of  cJiromosouies,  but  only  a  preliminary  ''  pseudo-reduction  "  in 
the  number  of  iz\v:ox\\2X\Yv-masses .  In  what  we  may  regard  as  the 
typical  case  {e.g.  Ascaris)  the  pseudo-reduction  first  appears  at  the 
penultimate  division ;  i.e.  in  the  grandmother-cell  of  the  germ-cell 
(primary  oocyte  or  spermatocyte).  It  may,  however,  appear  at  a  very 
much  earlier  period,  even  in  the  embryonic  germ-cells,  the  reduced 
number  appearing  in  every  succeeding  division  until  the  germ-cells 
are  formed.  This  is  the  case  in  the  salamander  and  in  Cyclops.  It 
appears  in  its  most  striking  form  in  the  higher  plants,  where  the  re- 


jo6 


REDUCTION  OF  THE    CHROMOSOMES 


T      r-r      Vil 


^ 


duced  number  appears  in  all  the  cells  of  the  sexual  generation  (pro- 
thallium,  pollen-tube,  embryo-sac),  beginning  with  the  mother-cell  of 
the  asexual  spores  from  which  this  generation  arises. 

In  every  case  we  must  distinguish  carefully  between  the  primary 
pseudo-reduction  in  the  number  of  chromatin-masses,  and  the  actual 
reduction  in  the  number  of  chromosomes ;  for  the  former  is  in  some 
cases  certainly  not  an  actual  halving  of  the  number  of  cJiTomosomes, 
since  each  of  the  primary  chromatin-rods  is  proved  by  its  later  history 
to  be  bivalent,  representing  two  chromosomes  united  end  to  end  (sal- 
amander, copepods).     In  these  cases  the  actual  reduction  takes  place 

in  the  course  of  the  last  two 
divisions  (formation  of  the  polar 
bodies  and  of  the  spermatids), 
each  bivalent  chromatin-rod  di- 
viding transversely  into  the  two 
chromosomes  which  it  repre- 
sents, and  at  the  same  time 
(or  earlier)  splitting  lengthwise. 
Each  primary  rod  thus  gives 
rise  to  a  tetrad  consisting  of 
two  pairs  of  chromosomes  which, 
by  the  two  final  divisions,  are 
distributed  one  to  each  of  the 
four  resulting  cells.  In  the 
copepods  the  first  division  sepa- 
rates the  longitudinal  halves  of 
the  chromosomes  and  is  there- 
fore an  "equal  division  "  (Weis- 
mann).  The  second  division 
corresponds  with  the  transverse 
division  of  the  primary  rod,  and  therefore  is  the  "reducing  division  " 
postulated  by  Weismann. 

This  result  gives  a  perfectly  clear  conception  of  the  process  of 
actual  reduction  and  its  relation  to  the  preparatory  pseudo-reduction 
that  precedes  it.  It  has,  however,  been  absolutely  demonstrated  in 
only  two  groups  of  animals,  viz.  the  copepods  and  the  vertebrates 
(amphibia),  and  a  diametrically  opposite  result  has  been  reached  in 
the  case  of  Ascaris  (Boveri,  Hertwig,  Brauer)  and  in  the  plants  (Gui- 
gnard,Strasburger).  In  Ascaris  typical  tetrads  are  formed,  but  all 
observers  agree  that  they  arise  by  a  double  longitudinal  splitting  of 
the  original  chromatin-rod.  In  the  plants  no  tetrads  have  been  ob- 
served, but  the  precise  nature  of  the  maturation-divisions  is  still  in 
doubt. 

We   have   thus  two   diametrically  opposing   results.     In    the   one 


Fig.  102.  —  Diagram  contrasting  the  two 
modes  of  tetrad-formation. 

A.  Ascaris-type.  Double  longitudinal  split- 
ting of  the  primary  rod;  no  reduction  in  the 
number  of  granules  ("  ids").  B.  Copepod-type. 
A  longitudinal  followed  by  a  transverse  division 
of  the  primary  rod ;  the  number  of  granules 
halved  by  the  second  division. 


SUMMARY  AND   CONCLUSION  20/ 

case  the  primary  halving  in  number  is  a  pseudo-reduction,  and  each 
tetrad  arises  by  one  longitudinal  and  one  transverse  division  of 
a  bivalent  chromosome,  representing  two  different  regions  of  the 
spireme-thread  (Hacker,  vom  Rath,  Ruckert,  Weismann).  In  the 
other  case^the  primary  halving  appears  to  be  an  actual  reduction, 
and  if  tetrads  are  formed,  they  arise  {Ascajis)  by  a  double  longitudi- 
nal splitting  of  the  primary  rod,  and  all  of  its  four  derivatives  repre- 
sent the  same  region  of  the  spireme-thread.  Since  the  latter  consists 
primarily  of  a  single  series  of  granules  ("  ids "  of  Weismann,  or 
chromomeres),  by  the  fission  of  which  the  splitting  takes  place,  the 
difference  between  the  two  views  comes  to  this :  that  in  the  second 
case  the  four  chromosomes  of  each  tetrad  must  represent  identical 
groups  of  granules,  while  in  the  first  case  they  represent  two  differ- 
ent groups  (Fig.  102).  In  the  second  case  the  maturation-divisions 
cannot  cause  a  reduction  in  the  number  of  different  kinds  of  ids. 
In  the  first  case  the  number  of  ids  is  reduced  to  one-half  by  the 
second  division  by  which  the  second  polar  body  is  formed,  or 
by  which  two  spermatids  arise  from  the  daughter-spermatocyte 
(Ruckert,  Hacker,  vom  Rath). 

The  first  view  must  obviously  stand  or  fall  with  the  conception  of 
the  primary  chromatin-rods  as  bivalent  chromosomes.  That  this  is 
a  valid  conception  is  in  my  judgment  demonstrated  by  Brauer's 
remarkable  observations  on  Artemia ;  for  in  this  case  it  is  impossi- 
ble to  escape  the  conclusion  that  the  "chromosomes"  of  those 
parthenogenetic  embryos  in  which  the  number  is  halved  are  bivalent, 
—  i.e.  have  the  value  of  two  chromosomes  united  by  their  ends,  — 
and  they  lend  the  strongest  support  to  vom  Rath's  and  Hacker's 
hypothesis.  For  if  the  number  of  chromosomes  be  merely  the 
expression  of  a  formative  tendency,  like  the  power  of  crystalliza- 
tion, inherent  in  each  specific  kind  of  chromatin,  why  should  the 
chromatin  of  the  same  animal  differ  in  the  two  cases  though  derived 
from  the  same  source  in  both  }  Yet  if  the  cleavage-nucleus  arises 
from  eighty-four  dyads  the  same  number  of  chromatin-rods  appears 
in  all  later  stages ;  whereas  if  the  dyads  break  each  into  two  separate 
chromosomes  before  their  union,  the  number  is  thenceforward  one 
hundred  and  sixty-eight.  So  great  is  the  force  of  this  evidence  that 
I  think  we  must  still  hesitate  to  accept  the  results  thus  far  attained 
in  Ascaris  and  the  plants,  and  must  await  further  research  in  this 
direction.  Until  the  contradiction  is  cleared  up  the  problem  of 
reduction  remains  unsolved. 


208  REDUCTION   OF   THE    CHROMOSOMES 

APPENDIX 
I .   Accessory  Cells  of  the  Testis 

It  is  necessary  to  touch  here  on  the  nature  of  the  so-called  "  Sertoli-cells,"  or 
supporting  cells  of  the  testis  in  mammals,  partly  because  of  the  theoretical  signifi- 
cance attached  to  them  by  Minot,  partly  because  of  their  relations  to  the  question 
of  amitosis  in  the  testis.  In  the  seminiferous  tubules  of  the  mammalian  testis,  the 
parent-cells  of  the  spermatozoa  develop  from  the  periphery  inwards  towards  the 
lumen,  where  the  spermatozoa  are  finally  formed  and  set  free.  At  the  periphery  is 
a  layer  of  cells  next  the  basement-membrane,  having  flat,  oval  nuclei.  Within 
this,  the  cells  are  arranged  in  columns  alternating  more  or  less  regularly 
with  long,  clear  cells,  containing  large  nuclei.  The  latter  are  the  Sertoli-cells, 
or  supporting  cells ;  they  extend  nearly  through  from  the  basement-membrane  to 
the  lumen,  and  to  their  inner  ends  the  young  spermatozoa  are  attached  by  their 
heads,  and  there  complete  their  growth.  The  spermatozoa  are  developed  from  cells 
which  lie  in  columns  between  the  Sertoli-cells,  and  which  undoubtedly  represent 
spermatogonia,  spermatocytes,  and  spermatids,  though  their  precise  relationship  is, 
to  some  extent,  in  doubt.  The  innermost  of  these  cells,  next  the  lumen,  are  sperma- 
tids, which,  after  their  formation,  are  found  attached  to  the  Sertoli-cells,  and  are 
there  converted  into  spermatozoa  without  further  division.  The  deeper  cells  from 
which  they  arise  are  spermatocytes,  and  the  spermatogonia  lie  deeper  still,  being 
probably  represented  by  the  large,  rounded  cells. 

Two  entirely  different  interpretations  of  the  Sertoli-cells  were  advanced  as  long 
ago  as  1 87 1,  and  both  views  still  have  their  adherents.  Von  Ebner  ('71)  at  first 
regarded  the  Sertoli-cell  as  the  parent-cell  of  the  group  of  spermatozoa  attached  to  it, 
and  the  same  view  was  afterwards  especially  advocated  by  Biondi  ('85),  and  is  still 
maintained  by  Minot  ('92),  who  regards  the  nucleus  of  the  Sertoli-cell  as  the  physio- 
logical analogue  of  the  polar  bodies,  i.e.  as  containing  the  female  nuclear  substance 
('92.  p.  77).  According  to  the  opposing  view,  first  suggested  by  Merkel  ('71),  the 
Sertoli-cell  is  not  the  parent-cell,  but  a  nurse-cell,  the  spermatozoa  developing  from 
the  columns  of  rounded  cells,  and  becoming  secojidarily  attached  to  the  Sertoli-cell, 
which  serves  merely  as  a  support  and  a  means  of  conveying  nourishment  to  the 
growing  spermatozoa.  This  view  was  advocated  by  Brown  ('85),  and  especially  by 
Benda  ('87).  In  the  following  year  ('88),  von  Ebner  himself  abandoned  his  early 
hypothesis  and  strongly  advocated  Benda's  views,  adding  the  very  significant  result 
that  four  spermatids  arise  from  each  spermatocyte.,  precisely  as  was  afterwards 
shown  to  be  the  case  in  Ascaris,  etc.  The  very  careful  and  thorough  work  of 
Benda  and  von  Ebner  leaves  no  doubt,  in  my  opinion,  that  mammalian  spermato- 
genesis conforms,  in  its  main  outlines,  with  that  of  Ascaris,  the  salamander,  and 
other  forms,  and  that  Biondi's  views,  which  Minot  unfortunately  adopts,  are  without 
foundation.  If  this  be  the  case,  Minofs  theoretical  interpretation  of  the  Sertoli-cell 
as  the  physiological  equivalent  of  the  polar  bodies,  of  course  collapses. 

Various  other  attempts  have  been  made  to  discover  in  the  spermatogenesis  a 
casting  out  of  material  which  might  be  compared  with  the  polar  bodies,  but  these 
attempts  have  now  only  an  historical  interest.  Van  Beneden  and  Julin  sought  such 
material  in  the  "  residual  corpuscles  "  left  behind  in  the  division  of  the  sperm-forming 
Cii\\9,oi  Ascaris.  Other  authors  have  regarded  in  the  same  light  the  "Nebenkern" 
(Waldeyer)  and  the  "residual  globules"  (Lankester,  Brown)  thrown  off  by  the 
developing  spermatozoa  of  mammals.  All  of  these  views  are,  like  Minot's,  wide 
of  the  mark,  and  they  were  advanced  before  the  real  parallel  between  spermato- 
genesis and  ovogenesis  had  l)een  made  known  by  Plainer  and  Hertwig. 


APPENDIX  209 

2.    Amitosis  in  the  Early  Sex-Cells 

Whether  the  progenitors  of  the  germ-cells  ever  divide  amitotically  is  a  question 
of  high  theoretical  interest.  Numerous  observers  have  described  amitotic  division 
in  testis-cell^,  and  a  few  also  in  those  of  the  ovary.  The  recent  observations  of 
Meves  ('91),  vom  Rath  C93),  and  Preusse  C95),  leave  no  doubt  whatever  that 
such  divisions  occur  in  the  testis  of  many  animals.  Vom  Rath,  however,  maintains, 
after  an  extended  investigation,  that  all  cells  so  dividing  do  not  belong  in  the  cycle 
of  development  of  the  germ-cells  ('93,  p.  164)  ;  that  amitosis  occurs  only  in  the  sup- 
porting or  nutritive  cells  (Sertoli-cells,  etc.),  or  in  such  as  are  destined  to  degenerate, 
like  the  ''residual  bodies"  of  Van  Beneden.  Meves  has,  however,  produced  strong 
evidence  ('94)  that  in  the  salamander  the  spermatogonia  may,  in  the  autumn,  divide 
by  amitosis,  and  in  the  ensuing  spring  may  again  resume  the  process  of  mitotic 
division,  and  give  rise  to  functional  spermatozoa.  On  the  strength  of  these  observa- 
tions, Flemming  ('93)  himself  now  admits  the  possibility  that  amitosis  may  form 
part  of  a  normal  cycle  of  development,  and  Preusse  has  recently  shown  that  amitosis 
may  continue  through  several  generations  in  the  early  ovarian  cells  of  Hemiptera 
without  a  sign  of  degeneration. 

LITERATURE.     V 

Van  Beneden,  E.  —  Recherches   sur   la  maturation  de  Toeuf,  la  f^condation  et  la 

division  cellulaire  :  Arch.  Biol.,  W.     1883. 
Boveri,   Th.  —  Zellenstudien,    I.,   III.     yena,    1887-90.     See  also   "  Befruchtung " 

(List  IV.). 
Brauer,  A.  —  Zur  Kenntniss  der  Spermatogenese  von  Ascaris  inegalocephala :  Arch. 

tnik.  Anat.,  XLII.     1893. 
Id.  —  Zur  Kenntniss  der  Reifung  der  parthenogenetisch  sich  entwickelnden  Eies 

von  Artemia  Salina:  Arch.  mik.  Anat..,  XLIII.     1894. 
Hacker,  V.  —  Die  Vorstadien  der  Eireifung  (General  Review)  :  Arch.  mik.  Anat., 

XLV.  2.     1895. 
Hertwig,  0.  —  Vergleich  der  Ei-  und  Samenbildung  bei  Nematoden.     Eine  Grund- 

lage  fiir  cellulare  Streitfragen  :  Arch.  mik.  Anat.,  XXXVI.     1890. 
Mark,  E.  L.  — (See  List  IV.) 
Platner,  G.  —  Uber  die  Bedeutung  der  Richtungskorperchen  :  Biol.  Centralb.,  VIIL 

1889.     - 
vom    Rath,  0.  —  Zur   Kenntniss  der   Spermatogenese   von    Gryllotalpa  vulgaris: 

Arch   mik.  Anat..,  XL.'     1892. 
Id.  —  Neue  Beitrage  zur  Frage  der  Chromatinreduction  in  der  Samen-  und  Eireife  : 

Arch.  mik.  Anat.,  XLVI.     1895. 
Ruckert,  J.  —  Die  Chromatinreduktion  der  Chromosomenzahl  im  Entwicklungsgang 

der  Organismen :  Ergebn.  d.  Anat.  u.  Efitwick.,  III.     1893  (1894). 
Strasburger,  E. — Uber  periodische  Reduktion  der  Chromosomenzahl  im  Entwick- 
lungsgang der  Organismen :  Biol.  Centralb.jXW.     1894. 


CHAPTER  VI 

SOME   PROBLEMS   OF   CELL-ORGANIZATION 


"Wir  miissen  deshalb  den  lebendeu  Zellen,  abgesehen  von  der  Molecularstructur  der 
organischen  Verbindungen,  welche  sie  enthalt,  noch  eine  andere  und  in  anderer  Weise  com- 
plicirte  Structur  zuschreiben,  und  diese  es  ist,  welche  wir  mit  dem  Namen  Organization 
bezeichnen."  Brucke.i 


"Was  diese  Zelle  eigentlich  ist,  dariiber  existieren  sehr  verschiedene  Ansichten." 

Hackel.2 

The  remarkable  history  of  the  chromatic  substance  in  the  matura- 
tion of  the  germ-cells  forces  upon  our  attention  the  problem  of  the 
ultimate  morphological  organization  of  the  nucleus,  and  this  in  its 
turn  involves  our  whole  conception  of  protoplasm  and  the  cell.  The 
grosser  and  more  obvious  organization  is  revealed  to  us  by  the  micro- 
scope as  a  differentiation  of  its  substance  into  nucleus,  cytoplasm,  and 
centrosome.  But,  as  Strasburger  has  well  said,  it  would  indeed  be  a 
strange  accident  if  the  highest  powers  of  our  present  microscopes  had 
laid  bare  the  ultimate  organization  of  the  cell.  Briicke  insisted  more 
than  thirty  years  ago  that  protoplasm  must  possess  a  far  piore  com- 
plicated morphological  organization  than  is  revealed  to  us  in  the 
visible  structure  of  the  cell,  and  suggested  the  possible  existence  of 
vital  units  ranking  between  the  molecule  and  the  cell.  Many  biologi- 
cal thinkers  since  Briicke's  time  have  in  one  form  or  other  accepted 
this  conception,  which  indeed  lies  at  the  root  of  nearly  all  recent 
attempts  to  analyze  exhaustively  the  phenomena  of  cell-life.  I  shall 
make  no  attempt  to  review  the  a  priori  arguments  that  have  been 
urged  in  favour  of  this  conception,^  but  will  rather  inquire  what 
are  the  extreme  conclusions  justified  by  the  known  facts  of  cell- 
structure. 

1  Elementarorganismen,  i86i,  p.  386. 
'^  Anlhropogenie,  1891,  p.  104. 

^  For  an  exhaustive  review  of  the  suliject  see  Yves  Delage,  La  Structure  du  protoplasma, 
ct  les  theories  sur  Vheredite.     Paris,  1 895. 

210 


THE   NATURE    OF  CELL-ORGANS  211 


A.     The  Nature  of  Cell-organs 

The  cellos,  in  Briicke's  words,  an  elementary  organism^  which  may 
by  itself  perform  all  the  characteristic  operations  of  life,  as  is  the  case 
with  the  unicellular  organisms,  and  in  a  sense  also  with  the  germ- 
cells.  Even  when  the  cell  is  but  a  constituent  unit  of  a  higher  grade 
of  organization,  as  in  multicellular  forms,  it  is  no  less  truly  an  organ- 
ism, and  in  a  measure  leads  an  independent  life,  even  though  its 
functions  be  restricted  and  subordinated  to  the  common  life.  It  is 
true  that  the  earlier  conception  of  the  multicellular  body  as  a  colony 
of  one-celled  forms  cannot  be  accepted  without  certain  reservations.^ 
Nevertheless,  all  the  facts  at  our  command  indicate  that  the  tissue- 
cell  possesses  the  same  morphological  organization  as  the  egg-cell,  or 
the  protozoan,  and  the  same  fundamental  physiological  properties  as 
well.  Like  these  the  tissue-cell  has  its  differentiated  structural  parts 
or  organs,  and  we  have  now  to  inquire  how  these  cell-organs  are  to 
be  conceived. 

The  visible  organs  of  the  cell  fall  under  two  categories  according 
as  they  are  merely  temporary  structures,  formed  anew  in  each  suc- 
cessive cell-generation  out  of  the  common  structural  basis,  or  per- 
manent structures  whose  identity  is  never  lost  since  they  are  directly 
handed  on  by  division  from  cell  to  cell.  To  the  former  category 
belong,  in  general,  such  structures  as  cilia,  pseudopodia,  and  the 
like ;  to  the  latter,  the  nucleus,  probably  also  the  centrosome,  and 
the  plastids  of  plant-cells.  A  peculiar  interest  attaches  to  the  per- 
manent cell-organs.  Closely  inter-related  as  these  organs  are,  they 
nevertheless  have  a  remarkable  degree  of  morphological  indepen- 
dence. They  assimilate  food,  grow,  divide,  and  perform  their  own 
characteristic  actions  like  coexistent  but  independent  organisms,  of 
a  lower  grade  than  the  cell,  living  together  in  colonial  or  symbiotic 
association.  So  striking  is  this  morphological  and  physiological 
autonomy  in  the  case  of  the  green  plastids  or  chromatophores  that 
neither  botanists  nor  zoologists  are  as  yet  able  to  distinguish  with 
absolute  certainty  between  those  that  form  an  integral  part  of  the 
cell,  as  in  the  higher  green  plants,  and  those  that  are  actually  inde- 
pendent organisms  living  symbiotically  within  it,  as  is  probably  the 
case  with  the  yellow  cells  of  Radiolaria.  Even  so  acute  an  investi- 
gator as  Watase  ('93,  i)  has  not  hesitated  to  regard  the  nucleus 
itself  —  or  rather  the  chromosome  —  as  a  distinct  organism  living  in 
symbiotic  association  with  the  cytoplasm,  but  having  had,  in  an  his- 
torical sense,  a  different  origin.     It  is  but  a  short  step  from  this  con- 

1  Cf.  p.  41. 


212  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

elusion  to  the  view  that  the  centrosome,  too,  is  such  an  independent 
organism  and  that  the  cell  is  a  symbiotic  association  of  at  least  three 
dissimilar  living  beings !  Such  a  conception  would,  however,  as  I 
believe,  be  in  the  highest  degree  misleading,  even  if  with  Watase  we 
limit  it  to  the  nucleus  and  the  cytoplasm.  The  facts  point  rather  to 
the  conclusion  that  all  cell-organs  arise  as  differentiated  areas  in  the 
common  structural  basis  of  the  cell,  and  that  their  morphological 
character  is  the  outward  expression  of  localized  and  specific  forms  of 
metabolic  activity. 

It  is  certain  that  some  of  the  cell-organs  are  the  seat  of  specific 
chemical  changes.  Chromatin  (nuclein)  is  formed  only  in  the  nucleus. 
The  various  forms  of  plastids  have  specific  metabolic  powers,  giving 
rise  to  chlorophyll,  to  pigment,  or  to  starch,  according  to  their  nature. 
The  centrosome,  as  Biitschli,  Strasburger,  and  Heidenhain  have  in- 
sisted, possesses  a  specific  chemical  character  to  which  its  remarkable 
effect  on  the  cytoplasm  must  be  due.^  Even  in  regions  of  the  cyto- 
plasm not  differentiated  into  distinct  cell-organs  the  metabolic  activities 
may  show  specific  and  constant  localization,  as  shown  by  the  deposit 
of  zymogen-granules,  the  secretion  of  membranes,  the  formation  of 
muscle-fibres,  and  a  multitude  of  related  facts.  Physiologically, 
therefore,  no  line  of  demarcation  can  be  drawn  between  permanent 
cell-organs,  transient  cell-organs,  and  areas  of  the  cell-substance  that 
are  physiologically  specialized  but  not  yet  morphologically  differen- 
tiated into  organs.  When  we  turn  to  the  structural  relations  of  cell- 
organs,  we  find,  I  think,  reason  to  accept  the  same  conclusion  in  a 
morphological  sense.  The  subject  may  best  be  approached  by  a 
consideration  of  the  structural  basis  of  the  cell  and  the  morphologi- 
cal relations  between  nucleus  and  cytoplasm. 


B.     Structural  Basis  of  the  Cell 

It  has  been  pointed  out  in  Chapter  I.  that  the  ultimate  structural 
basis  of  the  cell  is  still  an  open  question ;  for  there  is  no  general 
agreement  as  to  the  configuration  of  the  protoplasmic  network,  and 
we  do  not  yet  know  whether  the  fibrillar  or  the  alveolar  structure  is 
the  more  fundamental.  This  question  is,  however,  of  minor  impor- 
tance as  compared  with  the  microsome-problem,  which  is,  I  think,  the 
most  fundamental  question  of  cell-morphology,  and  which  is  equally 
pressing  whatever  view  we  may  hold  regarding  the  configuration  of 
the  network. 

Are  the  granules  described  as  "  microsomes  "  accidental  and  non- 
essential bodies,  produced,  it  may  be,  by  the  coagulating  effects  of 

1  Cf.  p.  77. 


STRUCTURAL    BASIS    OF    THE    CELL  213 

the  reagents,  as  Fischer's  experiments  suggest  ?  Or  are  they  normal 
and  constant  morphological  elements  that  have  a  definite  significance 
in  the  life  of  the  cell  ?  It  is  certain  that  the  microsomes  are  not 
merely  nodes  of  the  network,  or  optical  sections  of  the  threads,  as 
the  earlier  authors  maintained  ;  for  the  fibrillae  may  often  be  seen  to 
consist  of  regular  rows  of  granules.  Van  Beneden  gave  the  first 
clear  description  of  the  microsomes  in  this  regard  in  the  following 
words :  "  I  have  often  had  occasion  to  note  facts  that  establish  the 
essential  identity  of  the  moniliform  fibrillae  and  the  homogeneous 
fibrillae  of  the  protoplasm.  In  my  opinion  every  fibrilla,  though  it 
appear  under  the  microscope  as  a  simple  line  devoid  of  varicosities, 
is  formed  at  the  expense  of  a  moniliform  fibril  composed  of  micro- 
somes connected  with  one  another  by  segments  of  uniting  fibrils."  ^ 
Again,  in  a  later  work  he  says  of  the  fibrils  of  the  astral  system  in 
Ascaris :  *' It  is  easy  to  see  that  the  achromatic  fibrils  are  monili- 
form, that  they  are  formed  of  microsomes  united  by  inter-fibrils."  ^ 
Similar  observations  have  been  made  by  many  later  writers.  In  the 
eggs  of  sea-urchins  and  annelids,  which  I  have  carefully  studied,  there 
is  no  doubt  that  after  some  reagents,  e.g.  sublimate-acetic,  picro- 
acetic,  chromo-formic,  the  entire  astral  system  has  exactly  the  struct- 
ure described  by  Van  Beneden  in  Ascaris.  Although  the  basal 
part  of  the  astral  ray  appears  like  a  continuous  fibre,  its  distal  part 
may  be  resolved  into  a  single  series  of  microsomes,  like  a  string  of 
beads,  which  passes  insensibly  into  the  cytoreticulum.  The  latter  is 
composed  of  irregular  rows  of  distinct  granules  which  stain  intensely 
blue  with  haematoxylin,  while  the  substance  in  which  they  are  em- 
bedded, left  unstained  by  haematoxylin,  is  colored  by  red  acid  aniline 
dyes,  such  as  Congo  red  or  acid  fuchsin. 

The  difficulty  is  to  determine  whether  this  appearance  represents 
the  normal  structure  or  is  produced  by  a  coagulation  and  partial  dis- 
organization of  the  threads  through  the  action  of  the  reagents.  A 
justifiable  scepticism  exists  in  regard  to  this  point ;  for  it  is  perfectly 
certain  that  such  coagulation-effects  actually  occur  in  the  proteids  of 
the  cell-substance,  and  that  some  of  the  granules  there  observed  have 
such  an  origin.  It  is  very  difficult  to  determine  this  point  in  the  case 
of  the  cyto-microsomes,  owing  to  their  extreme  minuteness.  The 
question  must,  therefore,  be  approached  indirectly  by  way  of  an 
examination  of  the  nucleus  and  its  relation  to  the  cytoplasm.  Here 
we  find  ourselves  on  more  certain  ground  and  are  able  to  make  an 
analysis  that  in  a  certain  measure  justifies  the  hypothesis  that  the  cyto- 
microsomes  may  be  true  morphological  elements  having  the  power  of 
growth  and  division  like  the  cell-organs  formed  by  their  aggregation. 

^'83,P-  576,577-  2'87,  p.  266. 


SOME   PROBLEMS   OF  CELL-ORGANIZATION 


I.    Nucleus  and  Cytoplasm 

From  the  time  of  the  earlier  writings  of  Frommann  ('65,  '6'f), 
Arnold  (^67),  Heitzmann  ('73)>  ^^d  Klein  ('78),  down  to  the  present, 
an  increasing  number  of  observers  have  held  that  the  nuclear  reticu- 
lum is  to  be  conceived  as  a  modification  of  the  same  structural  basis 
as  that  which  forms  the  cytoplasm.  The  latest  researches  indicate, 
indeed,  that  true  chromatin  (nuclein)  is  confined  to  the  nucleus.^ 
But  the  whole  weight  of  the  evidence  now  goes  to  show  that  the 
linin-network  is  of  the  same  nature,  both  chemically  and  physically, 
as  the  cyto-reticulum,  and  that  the  achromatic  nuclear  membrane  is 
formed  as  a  condensation  of  the  same  substance.  Many  investi- 
gators, among  whom  may  be  named  Frommann,  Leydig,  Klein,  Van 
Beneden,  and  Reinke,  have  described  the  threads  of  both  the  intra- 
and  extra-nuclear  network  as  terminating  in  the  nuclear  membrane ; 
and  the  membrane  itself  is  described  by  these  and  other  observers  as 
being  itself  reticular  in  structure,  and  by  some  (Van  Beneden)  as 
consisting  of  closely  crowded  microsomes  arranged  in  a  network. 
The  clearest  evidence  is,  however,  afforded  by  the  origin  of  the 
spindle-fibres  in  mitotic  division  ;  for  it  is  now  well  established  that 
these  may  be  formed  either  inside  or  outside  the  nucleus,  and 
there  is  a  pretty  general  agreement  among  cytologists,  with  the 
important  exception  of  Boveri,  that  both  spindle-fibres  and  astral 
rays  arise  by  a  direct  rearrangement  of  the  pre-existing  structures.^ 
At  the  close  of  mitosis  the  central  portion  of  the  spindle  appears 
always  to  give  rise  to  a  portion  of  the  cytoplasm  lying  between  the 
daughter-nuclei ;  and  in  the  division  of  the  egg  in  the  sea-urchin 
I  have  obtained  strong  evidence  that  the  spindle-fibres  are  directly 
resolved  into  a  portion  of  the  general  reticulum.  These  fibres  are 
in  this  case  formed  inside  the  nucleus  from  the  linin-network ;  and 
we  have  therefore  proof  positive  of  a  direct  genetic  continuity  be- 
tween the  latter  and  the  cytoplasmic  structures.  But  more  than  this, 
I  have  found  reason  to  conclude  that  in  this  case  a  considerable 
part  of  the  linin-network  is  derived  from  the  chtvinatin,  that  the 
entire  nuclear  reticulum  is  a  continuous  structure,  and  that  it  is  no 
more  than  a  specially  differentiated  area  of  the  general  cell-network 
('95,  2).  This  conclusion  finds,  I  believe,  a  very  strong  support  in 
the  studies  of  Van  Beneden,  Heidenhain,  and  Reinke  reviewed 
beyond  (p.  223) ;  but  the  bearing  of  these  only  becomes  plain  after 
considering  the  morphological  differentiations  of  the  nuclear  net- 
work and  its  transformations  during  mitosis. 

1  Cf.  Hammarsten  ('95). 

2  The  long-standing  dispute  as  to  tlie  origin  of  the  nuclear  membrane  (whether  nuclear 
or  cytoplasmic)  is  therefore  of  little  moment. 


MORPHOLOGICAL    COMPOSITION   OF    THE  NUCLEUS  215 

C.    Morphological  Composition  of  the  Nucleus 

I.    The  Ckrornatift 

{a)  Hypothesis  of  the  Individuality  of  the  Chromosomes.  —  It 
may  now  be  taken  as  a  well-established  fact  that  the  nucleus  is 
never  formed  de  novo,  but  always  arises  by  the  division  of  a  pre- 
existing nucleus.  In  the  typical  mode  of  division  by  mitosis  the 
chromatic  substance  is  resolved  into  a  group  of  chromosomes,  always 
the  same  in  form  and  number  in  a  given  species  of  cell,  and  having 
the  power  of  assimilation,  growth,  and  division,  as  if  they  were 
morphological  individuals  of  a  lower  order  than  the  nucleus.  That 
they  are  such  individuals  or  units  has  been  maintained  as  a  definite 
hypothesis,  especially  by  Rabl  and  Boveri.  As  a  result  of  a  careful 
study  of  mitosis  in  epithelial  cells  of  the  salamander,  Rabl  ('85) 
concluded  that  the  chromosomes  do  not  lose  their  individuality  at  the 
close  of  division,  but  persist  in  the  chromatic  reticulum  of  the  resting 
nucleus.  The  reticulum  arises  through  a  transformation  of  the 
chromosomes,  which  give  off  anastomizing  branches,  and  thus  give 
rise  to  the  appearance  of  a  network.  Their  loss  of  identity  is, 
however,  only  apparent.  They  come  into  view  again  at  the  ensuing 
division,  at  the  beginning  of  which  "the  chromatic  substance  flows 
back,  through  predetermined  paths,  into  the  primary  chromosome- 
bodies  "  (Kernfaden),  which  reappear  in  the  ensuing  spireme-stage  in 
nearly  or  quite  the  same  position  they  occupied  before.  Even  in 
the  resting  nucleus,  Rabl  believed  that  he  could  discover  traces  of 
the  chromosomes  in  the  configuration  of  the  network,  and  he  de- 
scribed the  nucleus  as  showing  a  distinct  polarity  having  a  "  pole  " 
corresponding  with  the  point  towards  which  the  apices  of  the  chro- 
mosomes converge  {i.e.  towards  the  centrosome),  and  an  "anti- 
pole" (Gegenpol)  at  the  opposite  point  {i.e.  towards  the  equator 
of  the  spindle)  (Fig.  17).  .  Rabl's  hypothesis  was  precisely 
formulated  and  ardently  advocated  by  Boveri  in  1887  and  1888, 
and  again  in  189 1,  on  the  ground  of  his  own  studies  and  those 
of  Van  Beneden  on  the  early  stages  of  Ascaris.  The  hypothesis 
was  supported  by  extremely  strong  evidence,  derived  especially  from 
a  study  of  abnormal  variations  in  the  early  development  of  Ascaris, 
the  force  of  which  has,  I  think,  been  underestimated  by  the  critics 
of  the  hypothesis.  Some  of  this  evidence  may  here  be  briefly 
reviewed.  In  some  cases,  through  a  miscarriage  of  the  mitotic 
mechanism,  one  or  both  of  the  chromosomes  destined  for  the  second 
polar  body  are  accidentally  left  in  the  ^gg.  These  chromosomes 
give   rise  in   the   Q.gg  to   a  reticular  nucleus,  indistinguishable  from 


i6 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


the  egg-nucleus.  At  a  later  period  this  nucleus  gives  rise  to 
the  same  number  of  chromosomes  as  those  that  entered  into  its 
formation ;  i.e.  either  one  or  two.  These  are  drawn  into  the 
equatorial  plate  along  with  those  derived  from  the  germ-nuclei,  and 
mitosis  proceeds  as  usual,  the  number  of  chromosomes  being,  how- 
ever, abnormally  increased  from  four  to  five  or  six  (Fig.  103  C,  D). 
Again,  the  two  chromosomes  left  in  the  ^gg  after  removal  of  the 


Fig.  103.  —  Evidence  of  the  individuality  of  the  chromosomes.  Abnormalities  in  the  fertiliza- 
tion of  Ascaris.     [BOVERI.] 

A.  The  two  chromosomes  of  the  egg-nucleus,  accidentally  separated,  have  given  rise  each  to  a 
reticular  nucleus  (9.  9)  ;  the  sperm-nucleus  below  ( cf).  B,  Later  stage  of  the  same,  a  single 
chromosome  in  each  egg-nucleus,  two  in  the  sperm-nucleus.  C.  An  egg  in  which  the  second 
polar  body  has  been  retained;  p.br-  the  two  chromosomes  arising  from  it,  ?  the  egg-chromo- 
somes, cf  the  sperm-chromosomes.     D.  Resulting  equatorial  plate  with  six  chromosomes. 

second  polar  body  may  accidentally  become  separated.  In  this 
case  each  chromosome  gives  rise  to  a  reticular  nucleus  of  half  the 
usual  size,  and  from  each  of  these  a  single  chromosome  is  afterwards 
formed  (Fig.  103,  A,B).  Finally,  it  sometimes  happens  that  the  two 
germ-nuclei  completely  fuse  while  in  the  reticular  state,  as  is  nor- 
mally the  case  in  sea-urchins  and  some  other  animals  (p.  153).  From 
the  cleavage-nucleus  thus  formed  arise  four  chromosomes. 


MORPHOLOGICAL    COMPOSITION  OF   THE  NUCLEUS 


21 


These  remarkable  observations  show  that  whatever  be  the  nufnber 
of  chromosomes  entering  into  the  formation  of  a  reticular  nncleus,  the 
same  niunber  afterzvards  issue  from  it  —  a  result  which  demonstrates 
that  the  number  of  chromosomes  is  not  due  merely  to  the  chemical 
composition  of  the  chromatin-substance,  but  to  a  morphological  organ- 
ization of  the  nucleus.  A  beautiful  confirmation  of  this  conclusion 
was  afterwards  made  by  Boveri  ('93,  '95,  i)  and  Morgan  ('95,  4) 
in  the  case  of  echinoderms,  by  rearing  larvae  from  enucleated  egg- 


Fig.  104.  —  Evidence  of  the  individuality  of  the  chromosomes  in  the  egg  of  Ascaris.  [BOVEKI.] 
E.  Anaphase  of  the  first  cleavage.  F.  Two-cell  stage  with  lobed  nuclei,  the  lobes  formed  by 
the  ends  of  the  chromosomes.  G.  Early  prophase  of  the  ensuing  division;  chromosomes  re-form- 
ing, centrosomes  dividing.  H.  Later  prophase,  the  chromosomes  lying  with  their  ends  in  the 
same  position  as  before ;  centrosomes  divided. 


fragments,  fertilized  by  a  single  spermatozoon  (p.  258).  All  the 
nuclei  of  such  larvae  contain  but  half  the  typical  number  of  chromo- 
somes, —  i.e.  nine  instead  of  eighteen,  —  since  all  are  descended 
from  one  germ-nucleus  instead  of  two ! 

Van  Beneden  and  Boveri  were  able,  furthermore,  to  demonstrate 
in  Ascaris  that  in  the  formation  of  the  spireme  the  chromosomes 
reappear  in  the  same  position  as  those  which  entered  into  the  forma- 
tion of  the  reticulum,  precisely  as  Rabl  maintained.     As   the   long 


2l8 


SOME  PROBLEMS   OE  CELL-ORGANIZATION 


chromosomes  diverge,  their  free  ends  are  always  turned  towards  the 
middle  plane  (Fig.  69),  and  upon  the  reconstruction  of  the  daughter- 
nuclei  these  ends  give  rise  to  corresponding  lobes  of  the  nucleus,  as 
in  Fig.  104,  which  persist  throughout  the  resting  state.  At  the  suc- 
ceeding division  the  chromosomes  reappear  exactly  in  the  same  posi- 


Fig.  105.  —  Independence  of  paternal  and  maternal  chromatin  in  the  segmenting  eggs  of 
Cyclops.      [A-C.  from  RUCKERT;    D.  from  Hacker.J 

A.  First  cleavage-figure  in  C.  stremius ;  complete  independence  of  paternal  and  maternal 
chromosomes.  B.  Resulting  2-cell  stage  with  double  nuclei.  C.  Second  cleavage  ;  chromosomes 
still  in  double  groups.     D.  Blastomeres  with  double  nuclei  from  the  8-cell  stage  of  C.  brevicor?iis. 


tion,  their  ends  lying  in  the  nuclear  lobes  as  befo7'e  {¥\g.  104,  G,  H).  On 
the  strength  of  these  facts  Boveri  concluded  that  the  chromosomes 
must  be  regarded  as  "individuals"  or  "elementary  organisms,"  that 
have  an  independent  existence  in  the  cell.  During  the  reconstruc- 
tion of  the  nucleus  they  send  forth  pseudopodia  which  anastomose  to 
form  a  network  in  which  their  identity  is  lost  to  view.     As  the  cell 


MORPHOLOGICAL    COMPOSITION  OF   THE  NUCLEUS  219 

prepares  for  division,  however,  the  chromosomes  contract,  withdraw 
their  processes,  and  return  to  their  "  resting  state,"  in  which  fission 
takes  place.  Applying  this  conclusion  to  the  fertilization  of  the  ^gg, 
Boveri  expressed  his  belief  that  "  we  may  identify  every  chromatic 
element  arising  from  a  resting  nucleus  with  a  definite  element  that 
entered  into  the  formation  of  that  nucleus,  from  which  the  remark- 
able conclusion  follows  that  in  all  cells  deidved  in  the  regiilai"  course 
of  division  from  the  fertilized  egg,  one-half  of  the  chromosomes  are  of 
strictly  paternal  origin,  the  other  half  of  mateimalT  ^ 

Boveri's  hypothesis  has  been  criticised  by  many  writers,  especially 
by  Hertwig,  Guignard,  and  Brauer,  and  I  myself  have  urged  some 
objections  to  it.  Recently,  however,  it  has  received  a  support  so 
strong  as  to  amount  almost  to  a  demonstration,  through  the  re- 
markable observations  of  Riickert,  Hacker,  Herla,  and  Zoja  on  the 
independence  of  the  paternal  and  maternal  chromosomes.  These 
observations,  already  referred  to  at  p.  156,  may  be  more  fully  reviewed 
at  this  point.  Hacker  ('92,  2)  first  showed  that  in  Cyclops  strenuns,  as 
in  Ascaris  and  other  forms,  the  germ-nuclei  do  not  fuse,  but  give  rise 
to  two  separate  groups  of  chromosomes  that  lie  side  by  side  near  the 
equator  of  the  cleavage-spindle.  In  the  two-cell  stage  (of  Cyclops 
tenttico7'7iis)  each  nucleus  consists  of  two  distinct  though  closely  united 
halves,  which  Hacker  believed  to  be  the  derivatives  of  the  two  respec- 
tive germ-nuclei.  The  truth  of  this  surmise  was  demonstrated  three 
years  later  by  Riickert  ('95,  3)  in  a  species  of  Cyclops,  likewise  identi- 
fied as  C  strenuus  {¥\g.  105).  The  number  of  chromosomes  in  each 
germ-nucleus  is  here  twelve.  Riickert  was  able  to  trace  the  pater- 
nal and  maternal  groups  of  daughter-chromosomes  not  only  into  the 
respective  halves  of  the  daughter-nuclei  of  the  two-cell  stage,  but 
into  later  cleavage-stages.  From  the  bilobed  nuclei  of  the  two-cell 
stage  arises,  in  each  cell,  a  double  spireme,  and  a  double  group  of 
chromosomes,  from  which  are  formed  bilobed  or  double  nuclei  in  the 
four-cell  stage.  This  process  is  repeated  at  the  next  cleavage,  and 
the  double  character  of  the  nuclei  was  in  many  cases  distinctly  recog- 
nizable at  a  late  stage  when  the  germ-layers  were  being  formed. 

Finally  Victor  Herla's  remarkable  observations  on  Ascaris  ('93) 
showed  that  in  Ascai'is  not  only  the  chromatin  of  the  germ-nuclei, 
but  also  the  paternal  and  maternal  chromosomes,  remain  perfectly 
distinct  as  far  as  the  twelve-cell  stage  —  certainly  a  brilliant  confirma- 
tion of  Boveri's  conclusion.  Just  how  far  the  distinction  is  main- 
tained is  still  uncertain,  but  Hacker's  and  Riickert's  observations 
give  some  ground  to  believe  that  it  may  persist  throughout  the 
entire  life  of  the  embryo.      Both  these  observers  have  shown  that 

1  '91,  p.  410. 


220 


SOME  PROBLEMS   OF  CELL-ORGANIZATION 


the  chromosomes  of  the  germinal  vesicle  appear  in  two  distinct 
groups,  and  Riickert  suggests  that  these  may  represent  the  paternal 
and  maternal  elements  that  have  remained  distinct  throughout  the 
entire  cycle  of  development,  even  down  to  the  formation  of  the  ^^'g ! 
When  to  these  facts  is  added  the  evidence  afforded  by  Brauer's 
beautiful  observations  on  Artemiuy  no  escape  is  left  from  the 
hypothesis  of  the  individuality  of  the  chromosomes  in  one  form  or 


Fig.  io6.  —  Hybrid  fertilization  of  the  egg  of  Ascaris  megalocephala,  var.  bivalens,  by  the  sper- 
matozoon of  var.  univalens.     [Herla.] 

A.  The  germ-nuclei  shortly  before  union.  B.  The  cleavage-figure  forming;  the  sperm-nucleus 
has  given  rise  to  one  cluomosome  (cf ),  the  egg-nucleus  to  two  (9).  C  Two-cell  stage  dividing, 
showing  the  three  chromosomes  in  each  cell.  D.  Twelve-cell  stage,  with  the  three  distinct  chro- 
mosomes still  shown  in  the  primordial  germ-cell  or  stem-cell. 


another,  even  though  we  admit  that  Boveri's  statement  may  have 
gone  somewhat  too  far.  The  only  question  is  how  to  state  the  facts 
without  introducing  obscure  conceptions  as  to  what  constitutes  an 
*'  individual."  It  is  almost  certain,  as  pointed  out  beyond  (p.  221),  that 
the  chromosomes  are  not  the  ultimate  units  of  nuclear  structure,  for 
they  arise  as  aggregations  of  chromatin-grains  that  have  likewise  the 
power  of  growth  and  division.     The  fact  remains  —  and  it  is  one  of 


MORPHOLOGICAL    COMPOSITION  OF   THE   NUCLEUS  221 

the  highest  significance  —  that  these  more  elementary  units  group 
themselves  into  definite  aggregates  of  a  higher  order  that  show  a 
certain  degree  of  persistent  individual  existence.  It  may  be  said 
that  the  tejidency  to  assume  such  a  grouping  is  merely  a  question 
of  nuclear  dynamics,  and  is  due  to  a  "  formative  force  "  innate  in 
the  chromatin-substance.  This  is  undoubtedly  true ;  but  it  is  only 
another  form  of  expression  for  the  facts,  though  one  that  avoids  the 
use  of  the  quasi-metaphysical  term  "  individual."  Whether  a  chro- 
mosome that  emerges  from  the  resting  nucleus  is  individually  the 
same  as  one  that  entered  into  it  can  only  be  determined  when  we 
know  whether  it  consists  of  the  same  group  of  chromatin-granules 
or  other  elementary  bodies.  It  must  not  be  forgotten,  however,  that 
in  the  case  of  the  Q,gg  the  chromosomes  may  persist  without  loss  of 
their  boundaries  from  one  division  to  another,  since  no  reticulum  is 
formed  (cf.  p.  193). 

{b)  Composition  of  the  Chromosomes.  —  We  owe  to  Roux  ^  the  first 
clear  formulation  of  the  view  that  the  chromosomes,  or  the  chro- 
matin-thread,  consist  of  successive  regions  or  elements  that  are 
qualitatively  different  (p.  183).  This  hypothesis,  which  has  been 
accepted  by  Weismann,  Strasburger,  and  a  number  of  others,  lends 
a  peculiar  interest  to  the  morphological  composition  of  the  chromatic 
substance.  The  facts  are  now  well  established  (i)  that  in  a  large 
number  of  cases  the  chromatin-thread  consists  of  a  series  of  granules 
(chromomeres)  embedded  in  and  held  together  by  the  linin-substance, 
(2)  that  the  splitting  of  the  chromosomes  is  caused  by  the  division 
of  these  more  elementary  bodies,  (3)  that  the  chromatin-grains  may 
divide  at  a  time  when  the  spireme  is  only  just  beginning  to  emerge 
from  the  reticulum  of  the  resting  nucleus.  These  facts  point  unmis- 
takably to  the  conclusion  that  these  granules  are  perhaps  to  be  re- 
garded as  independent  morphological  elements  of  a  lower  grade  than 
the  chromosomes.  That  they  are  not  artefacts  or  coagulation-products 
is  proved  by  their  uniform  size  and  regular  arrangement  in  the  thread, 
especially  when  the  thread  is  split.  A  decisive  test  of  their  morpholog- 
ical nature  is,  however,  even  more  difficult  than  in  the  case  of  the  chro- 
mosomes ;  for  the  chromatin-grains  often  become  apparently  fused 
together  so  that  the  chromatin-thread  appears  perfectly  homogeneous, 
and  whether  they  lose  their  individuality  in  this  close  union  is  unde- 
termined. Observations  on  their  number  are  still  very  scanty,  but 
they  point  to  some  very  interesting  conclusions.  In  Boveri's  figures 
of  the  egg-maturation  of  Ascaris  each  element  of  the  tetrad  consists 
of  six  chromatin-disks  arranged  in  a  linear  series  (Van  Beneden's 
figures  of  the  same  object  show  at  most  five)  which  finally  fuse  to 

1  Bedetitung  der  Kerntheilungsjiguren^  1883,  p.  15. 


222  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

form  an  apparently  homogeneous  body.  In  the  chromosomes  of 
the  germ-nuclei  the  number  is  at  least  double  this  (Van  Beneden). 
Their  number  has  been  more  carefully  followed  out  in  the  sperma- 
togenesis of  the  same  animal  (variety  bivaiens)  by  Brauer.  At  the 
time  the  chromatin-grains  divide,  in  the  reticulum  of  the  spermato- 
cyte-nucleus,  they  are  very  numerous.  His  figures  of  the  spiremc- 
thread  show  at  first  nearly  forty  granules  in  linear  series  (Fig.  92,  A). 
Just  before  the  breaking  of  the  thread  into  two  the  number  is 
reduced  to  ten  or  twelve  (Fig.  92,  C).  Just  after  the  division  to  form 
the  two  tetrads  the  number  is  four  or  five  (Fig.  92,  D),  which  finally 
fuse  into  a  homogeneous  body. 

It  is  certain,  therefore,  that  the  number  of  chromomeres  is  not  con- 
stant in  a  given  species,  but  it  is  a  significant  fact  that  in  Ascaris  the 
final  number,  before  fusion,  appears  to  be  nearly  the  same  (four  to 
six)  both  in  the  oogenesis  and  the  spermatogenesis.  The  facts  re- 
garding bivalent  and  plurivalent  chromosomes  (p.  61)  at  once  sug- 
gest themselves,  and  one  cannot  avoid  the  thought  that  the  smallest 
chromatin-grains  may  successively  group  themselves  in  larger  and 
larger  combinations  of  which  the  final  term  is  the  chromosome. 
Whether  these  combinations  are  to  be  regarded  as  "individuals"  is 
a  question  which  can  only  lead  to  a  barren  play  of  words.  The  fact 
that  cannot  be  escaped  is  that  the  history  of  the  chromatin-substance 
reveals  to  us,  not  a  homogeneous  substance,  but  a  definite  morpho- 
logical organization  in  which,  as  through  an  inverted  telescope,  we 
behold  a  series  of  more  and  more  elementary  groups,  the  last  visi- 
ble term  of  which  is  the  smallest  chromatin-granule,  or  nuclear 
microsome  beyond  which  our  present  optical  appliances  do  not  allow 
us  to  see.  Are  these  the  ultimate  dividing  units,  as  Brauer  suggests 
(P-  79)  •''  Here  again  we  may  well  recall  Strasburger's  warning,  and 
hesitate  to  identify  the  end  of  the  series  with  the  limits  reached  by 
our  best  lenses.  Somewhere,  however,  the  series  must  end  in  final 
chromatic  units  which  cannot  be  further  subdivided  without  the 
decomposition  of  chromatin  into  simpler  chemical  substances.  These 
units  must  be  capable  of  assimilation,  growth,  and  division  without 
loss  of  their  specific  character.  This  I  believe  is  an  absolute  logical 
necessity.  It  is  in  these  ultimate  units  that  we  must  seek  the 
"qualities,"  if  they  exist,  postulated  in  Roux's  hypothesis;  but  the 
existence  of  such  qualitative  differences  is  a  physiological  assump- 
tion that  in  no  manner  prejudices  our  conclusion  regarding  the 
ultimate  morphological  composition  of  the  chromatin. 


CHROMATIN,   LININ,   AND    THE    CYTORETICULUM  223 


D.     Chromatin,   Linin,  and  the  Cytoreticulum 

What,  now,  is  the  relation  of  the  smallest  visible  chromatin-grains 
to  the  liniri-network  and  the  cytoreticulum  ?  Van  Beneden  long 
ago  maintained  ^  that  the  achromatic  network,  the  nuclear  mem- 
brane, and  the  cytoreticulum  have  essentially  the  same  structure, 
all  consisting  of  microsomes  united  by  connective  substance,  and 
being  only  "  parts  of  one  and  the  same  structure."  But,  more  than 
this,  he  asserted  that  tJie  cJwoinatic  and  achromatic  microsomes  might 
be  transformed  into  07te  another,  ajid  were  therefore  of  essentially  the 
same  morphological  nature.  "  They  pass  successively,  in  the  course 
of  the  nuclear  evolution,  through  a  chromatic  or  an  achromatic 
stage,  according  as  they  imbibe  or  give  off  the  chromophilous 
substance."  "^  Both  these  conclusions  are  borne  out  by  recent 
researches.  Heidenhain  ('93,  '94),  confirmed  by  Reinke  and  Schlo- 
ter,  finds  that  the  nuclear  network  contains  granules  of  two 
kinds  differing  in  their  staining-capacity.  The  first  are  the  basi- 
chromatin  granules,  which  stain  with  the  true  nuclear  dyes  (basic 
anilines,  etc.),  and  are  identical  with  the  *' chromatin-granules  "  of 
other  authors.  The  second  are  the  oxychromatin-granules  of  the 
linin-network,  which  stain  with  the  plasma-stains  (acid  anilines,  etc.), 
and  are  closely  similar  to  those  of  the  cytoreticulum.  Tliese  two 
forms  gi'adnate  into  one  another,  and  ai'e  conjectured  to  be  diffei^ent 
phases  of  the  same  elements.  This  conception  is  furthermore  sup- 
ported by  many  observations  on  the  behaviour  of  the  nuclear  net- 
work as  a  whole.  The  chromatic  substance  is  known  to  undergo 
very  great  changes  in  staining-capacity  at  different  periods  in  the 
life  of  the  nucleus  (p.  244),  and  is  known  to  vary  greatly  in  bulk. 
In  certain  cases  a  very  large  amount  of  the  original  chromatic  net- 
work is  cast  out  of  the  nucleus  at  the  time  of  the  division,  and  is 
converted  into  cytoplasm.  And,  finally,  in  studying  mitosis  in  sea- 
urchin  eggs  I  was  forced  to  the  conclusion  ('95,  2)  that  a  consid- 
erable part  of  the  linin-network,  from  which  the  spindle-fibres  are 
formed,  is  actually  derived  from  the  chromatin. 

When  all  these  facts  are  placed  in  connection,  we  find  it  difficult  to 
escape  the  conclusion  that  no  definite  line  can  be  drawn  between 
the  cytoplasmic  microsomes  at  one  extreme  and  the  chromatin-gran- 
ules  at  the  other.  And  inasmuch  as  the  latter  are  certainly  capable 
of  growth  and  division,  we  cannot  deny  the  possibility  that  the  former 
may  have  like  powers.  It  may  well  be  that  our  present  reagents  do 
not  give  us  a  true  picture  of  these  elementary  units — that  **  micro- 
somes "  are  but  a  rude  semblance  of  reality.     That  they  are  never- 

1  '83,  p,  580,  583.  ^  I.e.,  p.  583. 


224  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

theless  an  expression  of  the  morphological  aggregation  of  the  proto- 
plasmic network  out  of  more  elementary  units,  must,  I  think,  be 
accepted  as  a  working  hypothesis.  Whether  they  are  elementary 
organisms  in  Altmann's  sense,  whether  they  have  a  persistent  mor- 
phological identity,  whether  they  arise  solely  by  the  division  of  pre- 
existing microsomes,  or  may  undergo  dissolution  and  reformation, 
whether,  in  short,  they  are  the  self-propagating  elementary  bodies 
postulated  by  so  many  eminent  naturalists  as  the  essential  basis  of 
the  cell,  —  all  these  are  entirely  open  questions  which  the  cytology 
of  the  future  has  to  solve. 

E.     The  Centrosome 

When  we  turn  to  the  centrosome,  we  find  clear  evidence  of  the 
existence  of  a  cell-organ  which,  though  scarcely  larger  than  a  cyto- 
microsome,  possesses  specific  physiological  powers,  assimilates,  grows, 
divides,  and  may  persist  from  cell  to  cell,  without  loss  of  identity. 
It  is  far  easier  to  define  the  centrosome  in  physiological  than  in  mor- 
phological terms.  In  the  former  sense  Boveri  ('95,  2)  defines  it  as  a 
single  permmient  cell-or-gmi  which  forms  tJie  dynamic  centre  of  the  cell 
and  multiplies  by  division  to  form  the  centres  of  the  daiigliter-cells} 
A  centrosome  is  necessarily  present  in  all  cells  at  the  time  of  mitosis. 
Whether,  however,  it  persists  in  the  resting  state  of  all  cells  is  un- 
known. The  most  careful  search  has  thus  far  failed  to  reveal  its 
presence  in  many  tissue-cells,  e.g.  in  muscle-cells  and  many  gland- 
cells  ;  but  these  same  cells  may,  under  certain  conditions,  divide  by 
mitosis,  as  in  regeneration  or  tumour-formation,  and  the  centrosome 
may  be  hidden  in  the  nucleus,  or  so  minute  as  to  escape  observation. 
We  must,  however,  remember  that  the  centrosome  often  disappears 
in  the  mature  ^gg,  and  the  same  may  be  true  of  some  tissue-cells. 

Van  Beneden's  and  Boveri's  independent  identification  of  centrosome  in  Ascaris 
as  a  permanent  cell-organ  ('87)  was  quickly  supported  by  numerous  observations  on 
other  animals  and  on  plants.  In  rapid  succession  the  centrosome  and  attraction- 
sphere  were  found  to  be  present  in  pigment-cells  of  fishes  (Solger,  '89,  '90),  in  the 
spermatocytes  of  Amphibia  (Hermann,  '90),  in  the  leucocytes,  endothelial  cells,  con- 
nective tissue-cells  and  lung-epithelium  of  salamanders  (Flemming,  '91),  in  various 
plant-cells  (Guignard,  "'91),  in  the  one-celled  diatoms  (Biitschli,  '91),  in  the  giant- 
cells  and  other  cells  of  bone-marrow  (Heidenhain,  Van  Bambeke,  Van  der  Stricht, 
'91),  in  the  flagellate  Noctiluca  (Ishikawa,  '91),  in  the  cells  of  marine  algae  (Stras- 
burger,  '92),  in  cartilage-cells  (Van  der  Stricht,  '92),  in  the  cells  of  cancerous  growths 
(epithelioma,  Lustig  and  Galeotti,  '92),  in  the  young  germ-cells  as  already  described, 
and  finally,  in  gland-cells  (vom  Rath,  '95),  and  in  nerve-cells  (Lenhossdk,  '95). 
They  have  not  yet  been  found  in  resting  muscle-cells. 

1  The  fact  that  the  centrosome  is  double  in  many  cells  does  not  conflict  with  this  defini- 
tion, for  the  doubling  is  obviously  a  precocious  preparation  for  the  ensuing  division. 


THE    CENTROSOME 


225 


The  earlier  observers  of  the  centrosome  always  found  it  lying  in  the  cytoplasm, 
outside  the  nucleus.  Almost  simultaneously,  in  1893,  three  investigators  indepen- 
dently discovered  it  inside  the  nucleus  of  the  resting  cell,  —  Wasielewsky,  in  the 
young  ovarian  eggs  (oogonia)  of  Ascaris ;  Brauer,  in  the  spermatocytes  of  the  same 
animal;  and^Karsten.  in  the  cells  of  a  plant,  Psilotmn  (Humphrey  states,  however, 
that  Karsten's  observations  were  erroneous).  Several  later  observers  have  described 
a  similar  intra-nuclear  origin  of  the  centrosome,  and  several  of  these  (Zimmermann, 
Lavdovsky,  K^-uten)  have  followed  Wasielewsky  in  locating  it  in  the  nucleolus. 
Evidence  against  this  latter  view  has  been  brought  forward,  especially  by  Humphrey 
and  Brauer.  The  latter  observer  found  both  nucleoli  and  centrosome  as  separate 
bodies  within  the  nucleus.     He  made    further   the    interesting   discovery   that   in 


n 


A 


\V- 


B 


D 


Fig.  107.  —  Mitosis  with  intra-nuclear  centrosome,  in  the  spermatocytes  of  Ascaris  megalO' 
cepliala,  var.  nnivalens.     [Braukr.] 

A.  Nucleus  containing  a  quadruple  group  or  tetrad  of  chromosomes  (/),  nucleolus  (w),  and 
centrosome  (^r).  B.  C.  Division  of  the  centrosome.  D.E.F.  G.  Formation  of  the  mitotic  figure, 
centrosomes  escaping  from  the  nucleus  in  G. 


Ascaris  the  ce?itrosome  lies,  in  one  variety  {nnivalens^  inside  the  nucleus,  in  the  other 
variety  {bivalens^  outside  —  a  fact  which  proves  that  its  position  is  non-essential 
(cf.  Figs.  92  and  107).  Oscar  and  Richard  Hertwig  maintain  that  the  intra-nuclear 
position  of  the  centrosome  is  the  more  primitive,  the  centrosome  having  been 
originally  differentiated  from  a  part  of  the  nuclear  substance.  This  view  is  based 
in  the  main  on  the  facts  of  mitosis  in  the  Infusoria,  where  the  whole  mitotic  figure 
appears  to  arise  within  the  nuclear  membrane  (cf.  p.  62). 


Whether  a  true  centrosome  may  ever  arise  cie  novo  is  likewise 
undetermined.  The  possibility  of  such  an  origin  has  been  conceded 
by  a  number  of  recent  writers,  among  them  Bijrger,  Watase,  Richard 
Hertwig,   Heidenhain,  and  Reinke.       The  latter  author  ('94)  would 


226  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

distinguish  in  the  cell,  besides  the  *'  primary  centres  "  or  centrosomes, 
secondary  and  tertiary  centres,  the  latter  being  single  microsomes 
formed  at  the  nodes  of  the  network.  By  the  successive  aggrega- 
tions of  the  latter  may  arise  the  secondary  and  primary  centres  as 
new  formations.  Watase  ('94)  advocates  a  somewhat  similar  view, 
and  states  that  he  has  observed  numerous  gradations  between  a  true 
aster  and  such  "tertiary  asters"  as  Reinke  describes.  Further  evi- 
dence in  the  same  direction  is  afforded  by  Morgan's  remarkable 
observations  on  the  formation  of  "artificial  asters"  in  unfertilized 
sea-urchin  eggs  which  have  lain  for  some  time  in  sea-water  ('96). 
Such  eggs  often  contain  numerous  asters,  each  of  which  contains  a 
body  resembling  a  centrosome.^  Beside  these  observations  must  be 
placed  those  of  Richard  Hertwig,  on  the  formation  of  an  amphiaster 
in  ripe  unfertilized  sea-urchin  eggs  (p.  159).  All  these  observations 
are  of  high  interest  in  their  bearing  on  the  historical  origin  of  the 
centrosome ;  but  they  do  not  prove  that  the  centrosome  of  the  nor- 
mal aster  ever  arises  by  free  formation.  On  the  whole,  the  evidence 
has  steadily  increased  that  the  centrosome  is  to  be  classed  among  the 
permanent  cell-organs ;  but  whether  it  ranks  with  the  nucleus  in  this 
regard  must  be  left  an  open  question. 

The  known  facts  are  still  too  scanty  to  enable  us  to  state  precisely 
what  a  centrosome  is  in  a  morphological  sense,  either  as  regards  its 
actual  structure  or  its  relation  to  other  parts  of  the  cell.  In  its  sim- 
plest form  (Fig.  108,  y^)  the  centrosome  appears  under  the  highest 
powers  as  nothing  more  than  a  single  granule  of  extraordinary 
minuteness  which  stains  intensely  with  iron-haematoxylin,  and  can 
scarcely  be  distinguished  from  the  cyto-microsomes  except  for  the 
fact  that  it  lies  at  the  focus  of  the  astral  rays.  In  this  form  it 
appears  at  the  centre  of  the  young  sperm-aster  in  various  animals 
—  for  example  in  the  sea-urchin  (Boveri),  in  ChcEtopteriLS  (Mead), 
and  in  Nereis?  In  almost  all  cases,  however,  the  centrosome  after- 
wards assumes  a  more  complex  structure  and  becomes  surrounded 
by  certain  envelopes,  the  relation  of  which,  on  the  one  hand,  to  the 
centrosome  and,  on  the  other  hand,  to  the  astral  rays  have  not  yet 
been  fully  cleared  up. 

Boveri,  whose  observations  have  been  confirmed  by  Brauer,  Hacker, 
and  others,  described  the  centrosome  in  the  cleavage-asters  of  Ascaris 
as  a  small  sphere  containing  a  minute  central  granule ;  and  Brauer's 
careful  studies  on  the  spermatogenesis  of  the  same  animal  showed 

^  I  have  had  the  privilege  of  examining  Professor  Morgan's  preparations,  and  can  confirm 
his  statement  that  these  eggs  contain  but  a  single  nucleus  and  hence  are  not  polyspermic. 

2  This  appearance  is  not  due  to  the  shrinkage  of  a  larger  and  more  complex  structure,  as 
some  authors  have  suggested;  for  in  Nereis  such  a  structure  —  i.e.  the  centrosphere  —  is 
afterwards  developed  around  the  centrosome. 


THE    CENTROSQME  22/ 

that  both  these  structures  are  persistent  and  that  division  of  the  sphere 
is  preceded  by  division  of  the  granule  (Fig.  107).  The  central  granule 
is  exactly  like  the  simple  centrosome  of  the  sperm-aster  as  described 
above,  bu^  we  do  not  yet  know  with  certainty  the  genesis  of  the 
sphere  surrounding  it,  and  hence  cannot  state  whether  this  is  part 
of  the  centrosome  proper  or  a  part  of  the  centrosphere  surrounding 
it.  The  former  view  is  adopted  by  Boveri,  who  suggests  the  word 
'' centriole  "  for  the  central  granule;  and,  according  to  his  observa- 
tions on  Ascaris  and  on  sea-urchins,  the  simple  centrosome  of  the 
original  sperm-aster  enlarges  to  form  the  sphere,  while  the  centriole 
afterwards  appears  within  it.  In  the  case  of  Thalasserna^  however, 
Griffin's  observations  leave  no  doubt  that  the  central  granule  per- 
sists in  its  original  form  from  its  first  appearance  in  the  sperm-aster 
through  every  stage  of  the  cleavage-amphiaster,  dividing  during  the 
early  anaphase  in  each  aster  and  giving  rise  to  the  centrosomes  of 
the  daughter-asters  in  which  it  again  appears  as  a  simple  granule  at 
the  focus  of  the  rays  without  a  trace  of  surrounding  envelopes 
(Fig.  73).  In  the  cleavage-amphiaster  it  is  surrounded  by  a  some- 
what vague,  rounded  mass  (apparently  representing  the  entire  "  cen- 
trosome "  of  Boveri  and  Brauer),  which  in  turn  lies  in  a  reticulated 
centrosphere,  from  which  the  rays  radiate.  Both  these  structures 
disappear  during  the  late  anaphase,  leaving  only  the  central  granule. 
Here,  therefore,  the  true  centrosome  certainly  corresponds  to  the 
central  granule  or  centriole ;  and  all  the  surrounding  structures  be- 
long to  the  centrosphere. 

As  soon  as  we  look  further  we  find  apparent  departures  from  this 
simple  type  of  centrosome.  In  leucocytes  Heidenhain  finds  at  the 
centre  of  the  centrosphere  not  one  or  two,  but  always  three,  and  some- 
times four,  granules,  which  he  conceives  as  centrosomes  forming  a 
central  group  or  microcentrum.  In  the  giant-cells  of  bone-marrow 
the  central  group  consists  of  a  very  large  number  (a  hundred  or 
more)  of  such  granules,  each  of  which  is  again  conceived  as  a 
"centrosome  "  (Fig.  1 1,  D).  In  the  sea-urchin  {Echinus)  Boveri  states 
that  the  original  simple  centrosome  of  the  sperm-aster  enlarges 
greatly  to  form  a  relatively  large,  well-defined  sphere  in  which 
appear  numerous  granules  (centrioles),  which  he  would  compare 
individually  with  the  elements  of  Heidenhain's  ''central  group." 
I  have  given  a  somewhat  similar  account  of  the  facts  in  Toxopneti- 
stes,  describing  the  centrosphere  as  a  reticulated  mass  derived  from 
an  original  granule  or  centrosome  at  the  focus  of  the  rays,^  and  many 

1  Professor  Boveri  informs  me  that  I  was  in  error  in  attributing  to  him  the  view  that  the 
entire  central  mass  of  the  aster — i.e.  the  centrosphere  —  here  represents  the  centrosome. 
The  large  spherical  centrosome  of  Echinus  is  surrounded  by  a  clear  area  which  he  regards 
as  the  centrosphere. 


228  SOME  PROBLEMS   OF  CELL-ORGANIZATION 

Other  investigators  have  been  unable  to  find  a  distinct  body  to  be 
identified  as  a  centrosome  within  the  centrosphere.  As  far  as  the 
sea-urchins  are  concerned,  there  is,  I  think,  good  reason  to  doubt  not 
only  my  own  former  conclusions,  but  also  those  of  Boveri.  Both  vom 
Rath  ('95,  2)  and  Hill  ('95)  find  at  the  centre  of  the  centrosphere  in 
sea-urchins  a  distinct  black  granule  (**  centrosome  "),  which  becomes 
double  in  the  early  anaphase  precisely  as  in  Thalassema.  More- 
over, Griffin's  studies  under  my  direction  show  that  the  minute  single 
centrosome  of  TJialassema  entirely  loses  its  staining-power  after  cer- 
tain reagents  and  only  comes  into  view  after  other  treatment.^  I  am 
now,' therefore,  inclined  to  believe  that  many  if  not  all  of  the  accounts 
asserting  the  absence  of  a  minute  central  centrosome  in  the  centro- 
sphere are  based  on  unsuitable  methods,  and  that  in  most  of  such 
cases,  if  not  in  all,  it  is  really  present. 

However  this  may  be,  it  is  now  certainly  known  that  the  centro- 
some is  in  some  cases  a  granule  so  small  as  to  be  almost  indistin- 
guishable from  the  microsomes  ;  that  in  this  form  it  is  able  to  organize 
the  surrounding  cytoplasm  into  the  astral  system ;  and  that  in  this 
form  it  may  be  handed  on  by  division  from  cell  to  cell.  It  may  well 
be  that  in  some  cases  such  a  centrosome  may  multiply  to  form  a  cen- 
tral group,  as  in  leucocytes  and  giant-cells ;  that  it  may  enlarge  to 
form  a  granular  or  reticular  sphere,  as  Boveri  describes;  and  that 
the  individual  granules  within  such  a  sphere  do  not  have  the  value 
of  centrosomes.  Such  secondary  morphological  modifications  do  not 
affect  the  physiological  significance  of  the  centrosome  as  a  perma- 
nent cell-organ,  but  they  have  an  important  bearing  on  the  question 
of  its  relation  to  the  other  constituents  of  the  cell. 

The  latter  question  has  not  been  definitely  answered.  Butschli, 
who  has  been  followed  by  Erlanger,  regards  the  centrosome  as  a 
small  differentiated  area  in  the  general  alveolar  structure ;  and  he 
describes  it  in  the  sea-urchin  as  actually  made  up  of  a  number  of 
minute  vesicles  (Fig.  8,  B).  Burger  ('92)  suggested  that  the  entire 
attraction-sphere  and  aster  arise  by  a  centripetal  movement  of  micro- 
somes to  form  a  radiating  group  the  centre  of  which  (centrosome)  is 
represented  by  a  condensed  mass  of  the  ground-substance.  Watase 
('93,  '94)  added  the  very  interesting  suggestion  that  tJic  centrosome  is 
itself  nothing  other  than  a  microsome  of  the  same  morphological 
nature  as  those  of  the  astral  rays  and  the  general  thread-work,  differ- 
ing from  them  only  in  size  and  in  its  peculiar  powers.^     Despite  the 

1  The  centrosome  disappears  after  fixation  with  sublimate-acetic,  but  is  perfectly  shown 
after  pure  sublimate  or  picro-acetic.     See  Science,  Jan.  lo,  1896. 

■^  The  microsome  is  conceived,  if  I  understand  Watase  rightly,  not  as  a  permanent  mor- 
phological body,  but  as  a  temporary  varicosity  of  the  thread,  which  may  lose  its  identity  in 
the  thread  and  reappear  when  the  thread  contracts.  The  centrosome  is  in  like  manner  not 
a  permanent  organ  like  the  nucleus,  but  a  temporary  body  formed  at  the  focus  of  the  astral 


THE  ARCHOPLASMIC  STRUCTURES  229 

ambiguity  of  the  word  ''microsome"  Watase's  suggestion  is  full  of 
interest,  indicating  as  it  does  that  the  centrosome  is  morphologically 
comparable  to  other  elementary  bodies  existing  in  the  cytoplasmic 
structure,  ^and  which,  minute  though  they  are,  may  have  specific 
chemical  and  physiological  properties. 

F.     The  Archoplasmic  Structures 
I.    Asters  and  Spmdle 

The  asters  and  attraction-spheres  have  a  special  interest  for  the 
study  of  cell-organs ;  for  these  are  structures  that  may  divide  and 
persist  from  cell  to  cell  or  may  lose  their  identity  and  reform  in  suc- 
cessive cell-generations,  and  we  may  here  trace  with  the  greatest 
clearness  the  origin  of  a  cell-organ  by  differentiation  out  of  the  struct- 
ural basis.  Two  sharply  opposing  views  of  these  structures  are  now 
held.  Boveri  i^%%,  2),  who  has  been  followed  in  a  measure  by  Stras- 
burger,  maintains  that  the  attraction-sphere  of  the  resting  cell  is  com- 
posed of  a  distinct  substance,  ''  aixJwplasm,''  consisting  of  granules 
or  microsomes  aggregated  about  the  centrosome  as  the  result  of  an 
attractive  force  exerted  by  the  latter.  From  the  material  of  the 
attraction-sphere  arises  the  entire  achromatic  figure,  including  both 
the  spindle-fibres  and  the  astral  rays,  and  these  have  nothing  to  do 
with  the  general  reticulum  of  the  cell.  They  grow  out  from  the 
attraction-sphere  into  the  reticulum  as  the  roots  of  a  plant  grow  into 
the  soil,  and  at  the  close  of  mitosis  are  again  withdrawn  into  the  cen- 
tral mass,  breaking  up  into  granules  meanwhile,  so  that  each  daugh- 
ter-cell receives  one-half  of  the  entire  archoplasmic  material  of  the 
parent-cell.  This  material  is,  however,  wholly  distinct  from  that  of 
the  general  reticulum,  not,  as  many  earlier  observers  have  maintained, 
identical  with  it.  Boveri  was  further  inclined  to  believe  that  the 
individual  granules  or  archoplasmic  microsomes  were  "  independent 
structures,  not  the  nodal  points  of  a  general  network,"  and  that  the 
archoplasmic   rays   arose  by  the  arrangement  of   these   granules  in 

rays.  Once  formed,  however,  it  may  long  persist  even  after  disappearance  of  the  aster  and 
serve  as  a  centre  of  formation  for  a  new  aster.  In  the  latter  case  the  astral  rays  are  con- 
ceived as  actual  derivatives  of  the  centrosome  which,  as  it  were,  spins  them  out  in  the  cyto- 
plasm. "  The  aster,  from  this  point  of  view,  may  be  considered  as  a  physiological  device 
for  concentrating  the  cytoplasmic  substance  in  a  form  which  can  be  spun  out  again  into 
filaments  in  the  direction  which  will  produce  a  definite  physiological  effect"  ('94,  p.  284). 
This  part  of  Watase's  conception  is,  on  the  whole,  I  think,  opposed  to  the  facts,  though  it 
certainly  explains  the  inpushing  of  the  nuclear  membrane  during  the  prophases  of  mitosis. 
It  is  im|)ossible  to  believe  that  the  rays  of  the  enormous  sperm-aster  are  developed  out  of 
the  minute  granule  at  their  centre  or  that  they  flow  back  into  it  at  the  close  of  division. 
The  centrosome  increases  in  size  during  the  formation  of  the  aster,  decreases  during  its 
disappearance,  which  is  the  reverse  of  what  the  hypothesis  demands.  Many  other  argu- 
ments in  the  same  direction  might  be  urged. 


230  SOME   PROBLEMS   OF  CELL-ORGANIZATION 

rows  without  loss  of  their  individuality.^  In  a  later  paper  on  the 
sea-urchin  ('95)  this  view  is  somewhat  modified  by  the  admission  that 
in  this  case  the  archoplasm  may  not  pre-exist  as  formed  material,  but 
that  the  rays  and  fibres  may  be  a  new  formation,  crystallizing,  as  it 
were,  out  of  the  protoplasm  about  the  centrosome  as  a  centre,^  but 
having  no  organic  relation  with  the  general  reticulum. 

Strong  evidence  against  the  archoplasm-theory  has  been  brought 
forward  by  many  investigators,  and  I  believe  it  to  be  in  principle 
untenable.  Nearly  all  recent  workers  have  accepted  in  one  form  or 
another  the  early  view  of  Biitschli,  Klein,  and  Van  Beneden  that  the 
astral  rays  and  spindle-fibres,  and  hence  the  attraction-sphere,  arise 
through  a  morphological  rearrangement  of  the  pre-existing  protoplas- 
mic network,  under  the  influence  of  the  centrosome.  Although  this 
view  may  be  traced  back  to  the  early  work  of  Fol  ('73)  and  Auerbach 
('74),  it  was  first  clearly  formulated  by  Butschli  ('76),  who  regarded 
the  aster  as  the  optical  expression  of  a  peculiar  physico-chemical 
alteration  of  the  protoplasm  primarily  caused  by  diffusion-currents 
converging  to  the  central  area  of  the  aster.^  An  essentially  similar 
view  is  maintained  jn  Biitschli's  recent  great  work  on  protoplasm,* 
the  astral  "  rays  "  being  regarded  as  nothing  more  than  the  meshes 
of  an  alveolar  structure  arranged  radially  about  the  centrosome  (Fig. 
8,  B).  The  fibrous  appearance  of  the  astral  rays  is  an  optical  delu- 
sion, for  they  are  not  fibres,  but  flat  lamellae  forming  the  walls  of 
elongated  closed  chambers.  This  view  has  more  recently  been  urged 
by  Reinke  and  Eismond. 

The  same  general  conception  of  the  aster  is  adopted  by  most  of 
those  who  accept  the  fibrillar  or  reticular  theory  of  protoplasm,  the 
astral  rays  and  spindle-fibres  being  regarded  as  actual  fibres  forming 
part  of  the  general  network.  One  of  the  first  to  frame  such  a  con- 
ception was  Klein  i^J^),  who  regarded  the  aster  as  due  to  **  a  radiar 
arrangement  of  what  corresponds  to  the  cell-substance,"  the  latter 
being  described  as  having  a  fibrillar  character.^  The  same  view  is 
advocated  by  Van  Beneden  in  1883.  With  Klein,  Heitzman,  and 
Frommann  he  accepted  the  view  that  the  intra-nuclear  and  extra- 
nuclear  networks  were  organically  connected,  and  maintained  that 
the  spindle-fibres  arose  from  both.^  ''The  star-like  rays  of  the  asters 
are  nothing  but  local  differentiations  of  the  protoplasmic  network.^ 
.  .  .     In  my  opinion  the  appearance  of    the  attraction-spheres,  the 

1  '88,  2,  p.  80.  ^  I.e.,  p.  40. 

*  For  a  very  careful  review  of  the  early  views  on  this  subject,  see  Mark,  Umax.,  i88i. 

*  '92,  2,  pp.  158-169. 
^  It  is  interesting  to  note  that  in  the  same  place  Klein  anticipated  the  theory  of  fibrillar 
contractility,  both  the  nuclear  and  the  cytoplasmic  reticulum  being  regarded  as  contractile 
(/.r.,  p.  417). 

'■■  '^3.  P-  592.  "  '83,  p.  576. 


THE  ARCHOPLASMIC  STRUCTURES  23 1 

polar  corpuscle  (centrosome)  and  the  rays  extending  from  it,  includ- 
ing the  achromatic  fibrils  of  the  spindle,  are  the  result  of  the  appear- 
ance in  the  egg-protoplasm  of  two  centres  of  attraction  comparable 
to  two  magnetic  poles.  This  appearance  leads  to  a  regular  arrange- 
ment of  the  reticulated  protoplasmic  fibrils  and  of  the  achromatic 
nuclear  substance  with  relation  to  the  centres,  in  the  same  way  that 
a  magnet  produces  the  stellate  arrangement  of  iron  filings."  ^ 

This  view  is  further* developed  in  Van  Beneden's  second  paper, 
published  jointly  with  Neyt  i^^j).  "The  spindle  is  nothing  but  a 
differentiated  portion  of  the  asters."  ^  The  aster  is  a  "radial  structure 
of  the  cell-protoplasm,  whence  results  the  image  designated  by  the 
name  of  aster."  ^  The  operations  of  cell-division  are  carried  out 
through  the  "  contractility  of  the  fibrillae  of  the  cell-protoplasm  and 
their  arrangement  in  a  kind  of  radial  muscular  system  composed  of 
antagonizing  groups."* 

An  essentially  similar  view  of  the  achromatic  figure  has  been 
advocated  by  many  later  workers.  Numerous  observers,  such  as 
Rabl,  Flemming,  Carnoy,  Watase,  Eismond,  Reinke,  etc.,  have  ob- 
served that  the  astral  fibres  branch  out  peripherally  into  the  general 
reticulum  and  become  perfectly  continuous  with  its  meshes.  This  is 
very  clearly  shown  in  the  formation  of  the  sperm-aster  about  the 
middle-piece  of  the  spermatozoon.  In  the  sea-urchin  {Toxopneiistes) 
the  formation  of  the  rays  from  the  cytoplasmic  reticulum  can  be  fol- 
lowed step  by  step,  and  there  can,  I  think,  be  no  doubt  that  the  astral 
rays  arise  by  a  direct  transformation  or  morphological  rearrangement 
of  the  pre-existing  structure,  and  that  they  extend  themselves  at  their 
outer  ends,  as  the  sperm-aster  moves  through  the  egg-substance,  by 
progressive  differentiation  out  of  this  reticulum.^  Once  formed,  how- 
ever, the  rays  may  possess  a  considerable  degree  of  persistence  and 
may  actively  elongate  by  growth.  Only  thus  can  we  explain  the 
pushing  in  of  the  nuclear  membrane  by  the  ingrowing  spindle-fibres 
during  the  prophases  of  mitosis  in  certain  forms  (p.  50)  and  the 
bending  of  the  rays  when  two  asters  collide,  as  recently  described  by 
Kostanecki  and  Wierzejski  ('96).  It  seems  certain,  furthermore,  that 
during  the  rotation  of  the  amphiaster  in  the  formation  of  the  polar 
bodies  (Fig.  71)  and  in  similar  cases,  the  spindle,  at  least,  moves  bodily. 
The  substance  of  the  spindle  or  of  the  asters  may,  moreover,  persist 
in  the  resting  cell,  after  the  close  of  mitosis,  as  the  attraction- 
sphere  or  paranucleus  (Nebenkern),  and  in  such  cases  the  term 
"  archoplasm  "  may  conveniently  be  retained  for  descriptive  purposes. 
To  regard  the  archoplasm  as  a  primary  and  independent  constituent 
of  the  cell  would,  however,  as  I  believe,  be  an  error. 

■^  '83*  p-  550.       '  i-c->  p.  263.       3  I.e.,  p.  275.       4  I.e.,  p.  280.       s  '95,  2,  p.  446. 


232  SOME  PROBLEMS   OE  CELL-ORGANIZATION 


2.    The  Attraction-spJicre 

The  foregoing  conception  of  the  asters  receives  a  strong  support 
from  the  study  of  the  attraction-sphere  in  resting  cells.  It  is  agreed 
by  all  observers  that  this  structure  is  derived  from  the  aster  of  the 
dividing  cell ;  but  there  is  still  no  general  agreement  regarding  its 
precise  mode  of  origin  from  the  aster,  and  the  subject  is  confused  by 
differences  in  the  terminology  of  different  authors.  There  are  some 
cases  in  which  the  entire  aster  persists  throughout  the  resting  cell 
(leucocytes,  connective  tissue-cells)  and  the  term  ''attraction-sphere" 
has  by  some  authors  been  applied  to  the  whole  structure.  As  origi- 
nally used  by  Van  Beneden,  however,^  the  word  was  applied  (in 
Ascaris)  not  to  the  entire  aster  but  only  to  its  central  portion  —  a 
spherical  mass  bounded  by  a  circle  of  microsomes  from  which  the 
astral  rays  proceed.  At  the  close  of  division  the  rays  fade  away  in 
the  general  network,  leaving  only  the  central  sphere  containing  the 
ccntrosome.  Boveri's  account  of  the  same  object  was  entirely  differ- 
ent; for  he  conceived  the  attraction-sphere  (*' archoplasm-sphere") 
of  the  resting  cell  as  representing  the  entire  aster,  the  rays  being 
withdrawn  towards  the  centrosome  and  breaking  up  into  a  mass  of 
granules.  Later  workers  have  proposed  different  terminologies,  which 
are  at  present  in  a  state  of  complete  confusion.  Fol  ('91)  proposed 
to  call  the  centrosome  the  astrocentre,  and  the  spherical  mass  sur- 
rounding it  (attraction-sphere  of  Van  Beneden)  the  asirospJicrc. 
Strasburger  accepted  the  latter  term  and  proposed  the  new  word 
"  centrosphere "  for  the  astrosphere  and  the  centrosome  taken  to- 
gether.2  This  terminology  has  been  accepted  by  most  botanists  and  by 
some  zoologists.  A  new  complication  was  introduced  by  Boveri  ('95), 
who  applied  the  word  ''astrosphere"  to  the  entire  aster  ^y^oXw^w^ 
of  the  centrosome,  in  which  sense  the  phrase  "astral  sphere"  had 
been  employed  by  Mark  in  1881.  The  word  "astrosphere"  has 
therefore  a  double  meaning  and  would  better  be  abandoned  in  favour 
of  Strasburger's  convenient  term  "  centrosphere,"  which  may  be 
understood  as  equivalent  to  the  "astrosphere"  of  Fol. 

As  regards  the  structure  of  the  centrosphere,  two  well-marked  types 
have  been  described.  In  one  of  these,  described  by  Van  Beneden  in 
Ascaris,  by  Heidenhain  in  leucocytes,  by  Driiner  and  Braus  in  divid- 
ing cells  of  amphibia,  the  centrosphere  has  a  radiate  structure,  being 
traversed  by  rays  which  stretch  between  the  centrosome  and  the 
peripheral  microsome-circle  (Figs.  34,  108,  G).  In  the  other  form, 
described  by  Vejdovsky  in  the  eggs  of  RliyncJielmis,  by  Solger  and 
Zimmermann  in  pigment-cells,  by  myself  in  sea-urchin  eggs  and  in 

^'83,  P- 548.  '^'92,  p.  51. 


THE  ARCHOPLASMIC  STRUCTURES 


233 


Nereis,  by  Riickert  in  Cyclops,  and  in  a  number  of  other  cases,  the 
centrosphere  has  a  non-radiate  reticular  structure  (Figs.  71,  108,  E). 
In  some  cases  no  centrosome  has  been  found  in  this  sphere  ;  but  for 
reasons  already  stated  (p.  228)  I  incline  to  believe  that  a  centrosome 
is  really  present. 

In  many,  if  not  in  all  cases  of  both  types,  the  sphere  consists  of  an 
outer  and  an  inner  zone,  the  latter  enclosing  the  centrosome ;  but  the 
relation  of  the  inner  zone  to  the  centrosome  still  remains,  in  a  meas- 


:^^ 


-mm^: 


//^  / 


Fig.  108.  —  Dingranis  illustrating  various  descriptions  of  centrosome  and  centrosphere. 
A.  Simplest  type;  only  a  minute  centrosome  at  the  focus  of  the  ravs  (sperm-aster  in  many 
forms).  B.  Rays  proceeding  directly  from  a  centrosome  of  considerable  size  within  which  is  a 
central  granule.  Example,  Brauer's  description  of  the  spermatocytes  oi  Ascaris.  C.  Rays  pro- 
ceeding from  a  clear  centrosphere  (astrosphere  of  Strasourger) ,  enclosing  a  centrosome  like 
the  last  but  with  no  central  granule  (in  flowering  plants  accordiuij  to  Guignard,  Strasburger, 
and  others).  D.  An  extremely  minute  centrosome  lying  in  the  middle  of  a  large  reticulated  cen- 
trosphere {eg-.  Hill's  description  of  the  speim-aster  in  scii-urchins  and  tunicates).  E.  Like  the 
last,  but  with  a  small  spherical  body  surrounding  the  centrosome  (examples,  the  eggs  of  Jhalas- 
seina  aud  Ncfe/s).  F.  No  centrosome  as  distinguished  from  the  reticulated  centrosphere.  Ex- 
amples in  the  pigment-cells  of  fishes  according  to  Zimmerman,  in  the  eggs  of  echinnderms  ac- 
cording to  Wilson  ;  many  similar  accounts  have  been  given,  but  all  are  open  to  question.  G.  In 
Ascaris,  according  to  Van  Beneden,  outside  the  centrosome  lie  the  cortical  and  medullary  zones 
of  the  attraction-sphere.  H.  The  same  according  to  Boveri.  The  centrosome  contains  a  cen- 
tral granule  or  centriole  (cf.  B.)  ;  outside  this  is  a  clear  zone  (medullary  zone  of  Van  Beneden), 
and  outside  this  a  vaguely  defined  granular  zone,  probably  corresponding  to  Van  Beneden's 
cortical  zone. 


ure,  in  doubt.  Van  Beneden  described  the  centrosphere  in  Ascaris 
as  consisting  of  an  outer  cortical  and  an  inner  viedullary  zone,  both 
of  which  were  conceived  as  only  a  modification  of  the  inner  region  of 
the  aster.  Boveri's  account  is  somewhat  different.  The  centrosome 
is  described  as  surrounded  by  a  clear  zone  ("  heller  Hof  "), —  probably 
corresponding  with  Van  Beneden's  "medullary  zone,"  —  while  the 
"  cortical  zone  "  of  the  latter  author  is  not  recognized  as  distinct  from 
the  aster  (or  archoplasm-sphere).     The  centrosome  itself  contains  a 


234 


SOME  PROBLEMS    OF   CELL-ORGANIZAl'ION 


minute  central  granule  or  centriole.  This  discrepancy  between  Boveri 
and  Van  Beneden  was  cleared  up  in  a  measure  by  Heidenhain's 
beautiful  studies  on  the  asters  in  leucocytes,  and  the  still  more 
thorough  later  work  of  Driiner  on  the  spermatocyte-divisions  of  the 
salamander.  In  leucocytes  (Fig.  35)  the  large  persistent  aster  has  at  its 
centre  a  well-marked  radial  sphere  bounded  by  a  circle  of  microsomes, 
as  described  by  Van  Beneden,  but  without  division  into  cortical  and 
medullary  zones.     The  astral  rays,  however,  show  indications  of  other 

circles  of  microsomes  lying  out- 
side the  centrosphere.  Driiner 
found  that  a  whole  series  of  such 
concentric  circles  might  exist  (in 
the  cell  shown  in  Fig.  109  no 
less  than  nine),  but  that  the  inner- 
most two  are  often  especially 
distinct,  so  as  to  mark  off  a  cen- 
trosphere composed  of  a  medul- 
lary and  a  cortical  zone  precisely 
as  described  by  Van  Beneden. 
These  observations  show  conclu- 
sively that  the  centrosphere  of 
the  radial  type  is  merely  the  inner- 
most portion  of  the  aster,  which 
acquires  an  apparent  boundary 
through  the  especial  development 
of  a  ring  of  microsomes.  And 
thus  Van  Beneden's  original  view 
is  confirmed,  that  not  only  the 
aster  as  a  whole,  but  also  the  centro- 
sphere, is  but  a  modified  area  of  the 
general  cytoplasmic  thread-work. 
Heidenhain  points  out  that  there  are  many  cases  —  for  instance, 
the  young  sperm-aster  —  in  which  there  is  at  first  no  clearly  marked 
central  sphere,  and  the  rays  proceed  outward  directly  from  the  centro- 
some.  The  sphere,  in  such  cases,  seems  to  arise  secondarily  through 
a  modification  of  the  inner  ends  of  the  astral  rays.  Heidenhain  there- 
fore concludes  that  the  centrosome  is  the  only  constant  element  in  the 
sphere,  the  latter  being  a  secondary  formation  and  not  entitled  to  rank 
as  a  persistent  cell-organ,  though  it  may  in  certain  cases  persist  and 
divide  like  the  centrosome.  Vom  Rath,  who  has  made  a  very  careful 
study  of  the  attraction-spheres  in  a  large  number  of  cells  among  both 
vertebrata  and  invertebrata,  arrives  at  a  nearly  similar  view,  though 
he  lays  greater  stress  on  the  differentiation  and  independence  of  the 
sphere.     In  asters  of  dividing  cells  he  could  find  in  many  cases  no 


Fig.  109.  —  bpermatogonium  of  salaman- 
der.    [DrOner.] 

The  nucleus  lies  below.  Above  is  the 
enormous  aster,  the  centrosome  at  its  centre, 
its  rays  showing  indications  of  nine  concentric 
circles  of  microsomes.  The  area  within  the 
second  circle  probably  represents  the  "  attrac- 
tion-sphere "  of  Van  Beneden. 


THE  ARCHOPLASMIC  STRUCTURES  2$$ 

limit  between  sphere  and  aster,  though  in  other  cases  it  is  distinctly 
present.  In  the  resting  cell,  on  the  other  hand,  the  boundary  of  the 
sphere  is  often  very  sharply  marked,  so  that  the  sphere  appears  as  a 
well-defined  spherical  body.  The  origin  of  such  a  definite  sphere  from 
the  aster  fias  not  been  very  definitely  determined,  but  Driiner's  obser- 
vations indicate  that  it  arises  in  the  manner  described  by  Van  Bene- 
den,  through  the  disappearance  of  the  more  peripheral  portions  of  the 
astral  rays.     It  is,  in  other  words,  the  persistent  centrosphere.^ 

The  genesis  of  the  reticular  type  of  centrosphere  is  not  so  well 
determined.  In  Nereis  the  aster  (maturation-asters,  sperm-aster) 
has  at  first  nothing  more  than  a  minute  centrosome  at  its  centre. 
This  becomes  surrounded  at  a  later  period  by  a  large  reticulated 
centrosphere,  showing  no  sign  of  radial  arrangement,  that  appears 
to  arise  by  a  transformation  of  the  inner  ends  of  the  astral  rays. 
A  nearly  similar  account  is  given  by  Hill  in  the  case  of  the  sperm- 
aster  in  Strongyloccntrotus  and  Phallusia.  In  these  latter  cases  the 
centrosphere  shows  no  differentiation  into  cortical  and  medullary 
zones.  In  TJialasserna  and  Nereis,  on  the  other  hand,  the  minute  cen- 
trosome becomes  surrounded  by  a  somewhat  vague  body  distinctly 
different  from  the  reticulum  of  the  outer  centrosphere,  and  this 
body  perhaps  represents  a  ''medullary  zone."  This  body,  with  the 
centrosome,  corresponds  very  nearly  to  the  "  centrosome  "  of  Ascaris 
with  its  "centriole"  or  central  granule  as  described  by  Boveri  and 
Brauer ;  but  in  Thalassema  Griffin's  observations  show  conclusively 
that  the  minute  central  granule  alone  is  the  centrosome,  and  that 
the  surrounding  body  does  not  persist  after  division.  I  cannot 
avoid  the  suspicion  that  the  body  described  by  Boveri  as  the 
"  centrosome "  in  Echinus  may  represent  this  medullary  region  of 
the  centrosphere,  and  that  he,  like  myself,  may  have  overlooked  the 
centrosome.  Nor  does  it  seem  impossible  that  the  "  centriole "  or 
central  granule  of  Ascaris  (Boveri,  Brauer)  may  likewise  represent 
the  true  centrosome.  These  questions  can  only  be  cleared  up  by 
further  investigation. 

To  sum  up  :  The  history  of  the  "  archoplasmic  "  structures  gives 
strong  ground  for  the  conclusion  that  attraction-spheres,  asters,  and 
spindle  are,  like  the  nucleus,  differentiations  of  the  general  cell-netwoi^k, 
zvhich  is,  as  it  were,  vioulded  by  the  centrosome  into  a  specific  form. 
If  this  be  well  founded,  the  word  "archoplasm"  has  no  significance 
save  in  a  topographical  or  descriptive  sense.  In  this  light  it  is  an 
interesting  fact  that  the  aster  or  attraction-sphere  may  either  persist 
and  divide,  like  a  permanent  cell-organ,  or  may  disappear  and  re-form 
in  successive  cell-generations. 

1  The  same  general  result  is  indicated  in  the  case  of  plants,  though  the  phenomena  have 
here  been  less  carefuUv  examined. 


!36  SOME  PROBLEMS   OF  CELL-ORGANIZATION 


G.     Summary  and  Conclusion 

A  minute  analysis  of  the  various  parts  of  the  cell  leads  to  the 
conclusion  that  all  cell-organs,  whether  temporary  or  "  permanent," 
are  local  differentiations  of  a  common  structural  basis.  Temporary 
organs,  such  as  cilia  or  pseudopodia,  are  formed  out  of  this  basis, 
persist  for  a  time,  and  finally  merge  their  identity  in  the  common 
basis  again.  Permanent  organs,  such  as  the  nucleus  or  centrosome, 
are  constant  areas  in  the  same  basis,  which  never  are  formed  de  novOy 
but  arise  by  the  division  of  pre-existing  areas  of  the  same  kind. 
These  two  extremes  are,  however,  connected  by  various  interme- 
diate gradations,  examples  of  which  are  the  contractile  vacuoles  of 
Protozoa,  which  belong  to  the  category  of  temporary  organs,  yet  in 
many  cases  are  handed  on  from  one  cell  to  another  by  fission, 
and  the  attraction-spheres  and  asters,  which  may  either  persist  from 
cell  to  cell  or  disappear  and  re-form  about  the  centrosome. 

The  facts  point  strongly  to  the  conclusion,  which  has  been  espe- 
cially urged  by  De  Vries  and  Wiesner,  that  in  many  if  not  in  all 
cases  the  division  of  cell-organs  is  in  the  last  analysis  brought  about 
by  the  division  of  more  elementary  masses  of  which  they  are  made 
up  ;  and  furthermore  that  the  degree  of  permanence  depends  on  tJie 
degree  of  cohesion  manifested  by  these  masses.  The  clearest  evi- 
dence in  this  direction  is  afforded  by  the  chromatic  substance  of  the 
nucleus,  the  division  of  which  does  not  take  place  as  a  single  mass- 
division,  but  through  the  fission  of  more  elementary  discrete  bodies 
of  which  it  consists  or  into  which  it  is  resolved  before  division. 
Several  orders  of  such  bodies  are  visible  in  the  dividing  nucleus, 
forming  a  series  of  which  the  highest  term  is  the  plurivalent  chro- 
mosome, the  lowest  the  smallest  visible  dividing  basichromatin-grains, 
while  the  intermediate  terms  are  formed  by  the  successive  aggrega- 
tion of  these  to  form  the  chromomeres  of  which  the  dividing  chromo- 
somes consist.  Whether  any  or  all  of  these  bodies  are  "individuals  " 
is  a  question  of  words.  The  facts  point,  however,  to  the  conclusion 
that  at  the  bottom  of  the  series  there  must  be  masses  that  cannot  be 
further  split  up  without  loss  of  their  characteristic  properties,  and 
which  form  the  elementary  morphological  units  of  the  nucleus. 

There  is  reason  to  believe  that  the  linin-network  is  likewise  com- 
posed of  minute  bodies,  the  oxychromatin-granules,  which  are  closely 
similar  in  appearance  to  the  smallest  chromatin-grains,  and  differ 
from  them  only  in  chemical  nature  as  shown  by  the  difference  of 
staining-power.  Whether  the  oxychromatin-granules  have  also  the 
power  of  growth  and  division  is  unknown  ;  but  if,  as  Van  Beneden 
and  Heidenhain  maintain,  the  basichromatin-  and  oxychromatin-gran- 


SUMMARY  AND    CONCLUSION  237 

ules  be  only  different  modifications  of  the  same  element,  a  presump- 
tion certainly  exists  that  they  have  such  powers.  Vvhen  we  extend 
this  comparison  to  the  cytoplasm,  the  ground  becomes  more  uncer- 
tain. It  seems  well  established  that  the  cytoreticulum  is  of  the  same 
nature  as  the  linin-network.  If  this  be  admitted,  we  are  led  to  accept 
on  a  priori  grounds  that  some  at  least  of  the  cytomicrosomes  are  not 
artefacts,  but  morphological  bodies  comparable  with  those  of  the  linin- 
and  chromatin  networks,  and  like  them  capable  of  growth  and  division. 
This  conclusion  is,  as  yet,  no  more  than  a  somewhat  doubtful  inference. 
In  the  centrosome,  however,  we  have  a  body,  no  larger  in  many  cases 
than  a  ''microsome,"  which  is  positively  known  to  be  a  persistent 
morphological  element,  having  the  power  of  growth,  division,  and 
persistence  in  the  daughter-cells.  Probably  these  powers  of  the  cen- 
trosome would  never  have  been  discovered  were  it  not  that  its  stain- 
ing-capacity  renders  it  conspicuous  and  its  position  at  the  focus  of 
the  astral  rays  isolates  it  for  observation.  When  we  consider  the 
analogy  between  the  centrosome  and  the  basichromatin-grains, 
when  we  recall  the  evidence  that  the  latter  graduate  into  the  oxy- 
chromatin-granules,  and  these  in  turn  into  the  cytomicrosomes,  we 
must  admit  that  Briicke's  cautious  suggestion  that  the  whole  cell 
might  be  a  congeries  of  self-propagating  units  of  a  lower  order  is 
to-day  not  entirely  without  the  support  of  facts. 


LITERATURE.     VI 

Van  Beneden,  E.  —  (See  List  IV.) 

Van  Beneden  and  Julin.  —  La  segmentation  chez  les  Ascidiens  et  ses  rapports  avec 

Porganisation  de  la  larve  :  Arch.  Biol.,  V.     1884. 
Boveri,  Th.  —  Zellenstudien.     (See  List  IV.) 

Briicke,  C.  —  Die  Elementarorganismen  :    Wiener  Sits.-Ber.,  XLIV.     1861 . 
Biitschli,  0.  —  Protoplasma.     (See  List  I.) 
Hacker,  V.  —  Uber  den  heutigen  Stand  der  Centrosomenfrage :    Verh.  d.  deutsc/i. 

Zoo  I.  Ges.     1894. 
Heidenhain,  M.  —  (See  List  I.) 
Herla,  V.  — Etude  des  variations  de  la  mitose  chez  Tascaride  megalocephale  :  Arch. 

Biol.,  XIII.     1893. 
Nussbaum,   M.  —  Uber  die  Teilbarkeit  der  lebendigen  Alaterie :  Arch.  inik.  Anat., 

XXVI.     1886. 
Rabl,  C.  —  Uber  Zellteilung  :  Morph.  Jahrl?.,  X.     1885. 
Riickert,  J.  —  (See  List  IV.) 

De  Vries,  H.  —  Intracellulare  Pangenesis:   ye/ia,  1889. 

Watase,  S.  —  Homology  of  the  Centrosome:   Jonrn.  Morph..,  VIII.  2.      1893. 
Id.  —  On  the  Nature  of  Cell-organization  :    Woods  Holl  Biol.  Lectures.     1893. 
Wiesner,  J.  —  Die  Elementarstruktur  und  das  Wachstum  der  lebenden    Substanz : 

Wien.,  1892. 
Wilson,  Edm.  B.  —  Archoplasm,  Centrosome,  and    Chromatin    in    the    Sea-urchin 

1895. 


CHAPTER   VII 

SOME   ASPECTS   OF   CELL-CHEiMISTRY   AND   CELL-PHYSIOLOGY 

"  Les  phenomenes  fonctionnels  ou  de  depense  vitale  auraient  done  leur  siege  dans  le 
protoplasme  celhdaire. 

"  Le  noyau  est  un  appareil  de  syntJiese  organique,  l^ instrument  de  la  production,  le  germe 
de  la  cellule:'  Claude  Bernard.! 

I 

A.     Chemical  Relations  of  Nucleus  and  Cytoplasm 

It  is  no  part  of  the  purpose  of  this  work  to  give  even  a  sketch  of 
general  cell-chemistry.  I  shall  only  attempt  to  consider  certain  ques- 
tions that  bear  directly  upon  the  functional  relations  of  nucleus  and 
cytoplasm  and  are  of  especial  interest  in  relation  to  the  process  of 
nutrition  and  through  it  to  the  problems  of  development.  It  has 
often  been  pointed  out  that  we  know  little  or  nothing  of  the  chemi- 
cal conditions  existing  in  living  protoplasm,  since  every  attempt  to 
examine  them  by  precise  methods  necessarily  kills  the  protoplasm. 
We  must,  therefore,  in  the  main  rest  content  with  inferences  based 
upon  the  chemical  behaviour  of  dead  cells.  But  even  here  investiga- 
tion is  beset  with  difficulties,  since  it  is  in  most  cases  impossible  to 
isolate  the  various  parts  of  the  cell  for  accurate  chemical  analysis, 
and  we  are  obliged  to  rely  largely  on  the  less  precise  method  of 
observing  with  the  microscope  the  visible  effects  of  dyes  and  other 
reagents.  This  difficulty  is  increased  by  the  fact  that  both  cytoplasm 
and  karyoplasm  are  not  simple  chemical  compounds,  but  mixtures  of 
many  complex  substances ;  and  both,  moreover,  undergo  periodic 
changes  of  a  complicated  character  which  differ  very  widely  in  dif- 
ferent kinds  of  cells.  Our  knowledge  is,  therefore,  still  fragmentary, 
and  we  have  as  yet  scarcely  passed  the  threshold  of  a  subject  which 
belongs  largely  to  the  cytology  of  the  future. 

It  has  been  shown  in  the  foregoing  chapter  that  all  the  parts  of 
the  cell  arise  as  local  differentiations  of  an  all-pervading  substratum 
which  in  the  greater  number  of  cases,  perhaps  in  all,  has  the  form  of 

1  Lemons  sur  les  phenomenes  de  la  vie,  T.,  1878,  p.  198. 
238 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM       239 

a  sponge-like  network.  Cell-organs,  such  as  the  nucleus,  the  spindle 
and  asters,  the  centrosome,  are  to  be  regarded  as  specialized  areas 
in  this  network,  just  as  the  visible  organs  of  the  multicellular  body 
are  specialized  regions  in  the  all-pervading  cellular  tissue.  And  pre- 
cisely as  the  various  organs  and  tissues  are  the  seat  of  special  chemi- 
cal activities  leading  to  the  formation  and  characteristic  transformation 
of  specific  substances,  —  as  for  instance  haemoglobin  is  characteristic 
of  the  red  blood-corpuscles,  or  chlorophyll  of  the  assimilating  tissues  of 
plants,  —  so  in  the  cell  the  various  morphological  regions  are  areas 
of  specific  chemical  activities  and  are  characterized  by  the  presence 
of  corresponding  substances.  The  morphological  differentiation  of 
cell-organs  is  therefore  in  a  way  the  visible  expression  of  underlying 
chemical  specializations ;  and  these  are  in  the  last  analysis  reducible 
to  differences  of  metabolic  action. 

I.     TJie  Proteids  and  their  Allies 

The  most  important  chemical  compounds  found  in  the  cell  are  the 
group  of  pi'otein  substances;  and  there  is  every  reason  to  believe  that 
these  form  the  principal  basis  of  living  protoplasm  in  all  of  its  forms. 
These  substances  are  complex  compounds  of  carbon,  hydrogen,  nitro- 
gen, and  oxygen,  often  containing  a  small  percentage  of  sulphur,  and 
in  some  cases  also  phosphorus  and  iron.  They  form  a  very  exten- 
sive group  of  which  the  different  members  differ  considerably  in 
physical  and  chemical  properties,  though  all  have  certain  common 
traits  and  are  closely  related.  They  are  variously  classified  even  by 
the  latest  writers.  Halliburton  ('93)  employs  the  word  "  proteids  " 
as  synonymous  with  albnniinoiis  substances,  including  under  them  the 
various  forms  of  albumin  (egg-albumin,  cell-albumin,  muscle-albumin, 
vegetable-albumins),  globulin  (fibrinogen,  vitellin,  etc.),  and  the  pep- 
tones (diffusible  hydrated  proteids).  This  author  places  in  a  sepa- 
rate class  of  albuminoids  another  series  of  nearly  related  substances 
(reckoned  by  some  chemists  among  the  '*  proteids "),  examples  of 
which  are  gelatine,  mucin,  and  especially  nnclein,  and  the  nucleo- 
albumins.  The  three  last-named  bodies  are  characterized  by  the 
presence  of  phosphorus,  in  which  respect  they  show  a  very  definite 
contrast  to  the  '*  proteids,"  many  of  which,  such  as  egg-albumin,  con- 
tain no  phosphorus,  and  others  only  a  trace.  By  Hammarsten  and 
some  others  the  word  "proteid"  is,  however,  employed  in  a  more 
restricted  sense,  being  applied  to  substances  such  as  the  nucleins 
and  nucleo-proteids,  of  greater  complexity  than  the  albumins  and 
globulins.  The  latter,  together  with  the  nucleo-albumins,  are  classed 
as  albuminous  bodies  (Eiweisskorper).^ 

1  See  Ilammersten,  '95,  p.  16. 


240      SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PIIYSIOLOGY 

The  distribution  of  these  substances  throughout  the  cell  varies 
greatly  not  only  in  different  cells,  but  at  different  periods  in  the  life 
of  the  same  cell.  The  cardinal  fact  always,  however,  remains,  that 
there  is  a  definite  and  constant  contrast  betivccn  ituclens  and  cytoplasm. 
The  latter  always  contains  large  quantities  of  nucleo-albumins,  certain 
globulins,  and  sometimes  small  quantities  of  albumins  and  peptones ; 
the  former  contains,  in  addition  to  these,  nuclcin  and  nncleo-protcids, 
which  as  the  names  indicate,  forms  its  main  bulk  and  its  most  con- 
stant and  characteristic  feature.  It  is  the  remarkable  substance, 
nuclein, — which  is  almost  certainly  identical  with  chromatin,  —  that 
chiefly  claims  our  attention  here  on  account  of  the  physiological  7'dle 
of  the  nucleus. 

2.    The  Nuclcin   Series 


Nuclein  was  first  isolated  and  named  by  Miescher  in  1 871,  by 
subjecting  cells  to  artificial  gastric  digestion.  The  cytoplasm  is 
thus  digested,  leaving  only  the  nuclei ;  and  in  some  cases,  for  in- 
stance pus-cells  and  spermatozoa,  it  is  possible  by  this  method  to 
procure  large  quantities  of  nuclear  substance  for  accurate  quanti- 
tative analysis.  The  results  of  analysis  show  it  to  be  a  complex 
albuminoid  substance,  rich  in  phosphorus,  for  which  Miescher  gave 
the  chemical  formula  C29H49NgP3022-  Later  analyses  gave  some- 
what discordant  results,  as  appears  in  the  following  table  of  per- 
centage-compositions :  ^  — 


Pits-cells. 

Spermatozoa  of  Salmon. 

Human  Brain. 

(Hovpe-Seyler.) 

(Miescher.) 

(v.  Jaksch.) 

c 

49.58 

36.11 

50.6 

H 

7.10 

5-15 

7.6 

N 

15.02 

13.09 

13.18 

P 

2.28 

5-59 

1.89 

These  differences  led  to  the  opinion,  first  expressed  by  Hoppe- 
Seyler,  and  confirmed  by  later  investigations,  that  there  are  several 
varieties  of  nuclein  which  form  a  group  having  certain  characters 
in  common.  Altmann  ('89)  opened  the  way  to  an  understanding 
of  the  matter  by  showing  that  "  nuclein  "  may  be  split  up  into  two 
substances;  namely,  (i)  an  organic  acid  rich  in  phosphorus,  to  which 
he  gave  the  name  nucleic  acid,  and  (2)  a  form  of  albumin.     Moreover, 

1  P>om  Halliburton,  '91,  p.  203.     [The  oxygen-percentage  is  omitted  in  this  table.] 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM       24 1 

the  nuclein  may  be  synthetically  formed  by  the  re-combination  of 
these  two  substances.  Pure  nucleic  acid  contains  no  sulphur,  a 
high  percentage  of  phosphorus  (above  9  %),  and  no  albumin.  By 
adding  it  to  a  solution  of  albumin  a  precipitate  is  formed  which 
contains  sulphur,  a  lower  percentage  of  phosphorus,  and  has  the 
chemical  characters  of  nuclein.  This  indicates  that  the  discord- 
ant results  in  the  analyses  of  nuclein,  referred  to  above,  were 
probably  due  to  varying  proportions  of  the  two  constituents ;  and 
Altmann  suggested  that  the  ''nuclein"  of  spermatozoa,  which  con- 
tains no  sulphur  and  a  maximum  of  phosphorus  (over  9.5  %),  might 
be  uncombined  nucleic  acid  itself.  Kossel  accordingly  drew  the 
conclusion,  based  on  his  own  work  as  well  as  that  of  Liebermann, 
Altmann,  Malfatti,  and  others,  'that  "  what  the  histologists  designate 
as  cJiromatiii  consists  essentially  of  combinations  of  nucleic  acid  with 
more  or  less  albumin,  and  in  some  cases  may  even  be  free  nucleic 
acid.  The  less  the  percentage  of  albumin  in  these  compounds,  the 
nearer  do  their  properties  approach  those  of  pure  nucleic  acid,  and 
we  may  assume  that  the  percentage  of  albumin  in  the  chromatin 
of  the  same  nucleus  may  vary  according  to  physiological  condi- 
tions." ^  In  the  same  year  Halliburton,  following  in  part  Hoppe- 
Seyler,  stated  the  same  view  as  follows.  The  so-called  "  nucleins  " 
form  a  series  leading  downward  from  nucleic  acid  thus  :  — 

(i)    Those  containing  no  albumin  and  a  maximum  (9-10  %)  of  phos- 
phorus (pure  nucleic  acid).     Nuclei  of  spermatozoa. 

(2)  Those  containing  little  albumin  and  rich  in  phosphorus.     Chro- 

matin of  ordinary  nuclei. 

(3)  Those  with  a  greater  proportion  of  albumin  —  a  series  of  sub- 

stances in  which  may  probably  be  included  py7'e7tm  (nucleoli) 
2iX\di  plastin  (linin).     These  graduate  into 

(4)  Those  containing  a  minimum   (0.5  to   i  %)  of  phosphorus  — 

the  nucleo-albumins,  which  occur  both  in  the  nucleus  and  in 
the  cytoplasm  (vitellin,  caseinogin,  etc.). 

Finally,  we  reach  the  globulins  and  albumins,  especially  character- 
istic of  the  cell-substance,  and  containing  no  nucleic  acid.  "  We  thus 
pass  by  a  gradual  transition  (from  the  nucleo-albumins)  to  the  other 
proteid  constituents  of  the  cell,  the  cell-globulins,  which  contain  no 
phosphorus  whatever,  and  to  the  products  of  cell-activity,  such  as 
the  proteids  of  serum  and  of  egg-white,  which  are  also  principally 
phosphorus-free."  2  Further,  "in  the  processes  of  vital  activity  there 
ar€  changing  relations  between  the  phosphorized  constituents  of  the 
nucleus,  just  as  in  all  metabolic  processes  there  is  a  continual  inter- 

i'93,  p.  158.  2  '93,  p.  574. 


242      SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

change,  some  constituents  being  elaborated,  others  breaking  down 
into  simpler  products."  ^  These  conclusions  established  a  probability 
that  the  chemical  differences  between  chromatin  and  cytoplasm, 
striking  and  constant  as  they  are,  are  differences  of  degree  only; 
and  they  opened  the  way  to  a  more  precise  investigation  of  the 
physiological  role  of  nucleus  and  cytoplasm  in  metabolism. 

3.    Staining-reactions  of  the  Nuclein-seiHes 

We  may  now  bring  these  facts  into  relation  with  the  staining- 
reactions  of  chromatin  and  cytoplasm  when  treated  with  the  aniline 
dyes.  These  dyes  are  divided  into  two  main  classes,^  viz,  the 
"  basic  "  anilines  and  the  "  acid  "  anilines,  the  colouring-matter  playing 
the  part  of  a  base  in  the  former  and  of  an  acid  in  the  latter.  The 
basic  anilines  {e.g.  methyl-green,  Bismarck  brown,  saffranin)  are  in 
general  "nuclear  stains,"  having  a  strong  affinity  for  chromatin, 
while  the  acid  anilines  (acid  fuchsin,  Congo  red,  eosin,  etc.)  are 
"plasma-stains,"  colouring  more  especially  the  cytoplasmic  elements. 
We  owe  to  Malfatti  and  Lilienfeld  the  very  interesting  discovery 
that  the  various  vteinbers  of  the  niiclein  series  shozv  an  affinity  for 
the  basic  dyes  in  direct  proportion  to  the  amount  of  nucleic  acid 
(as  meastired  by  the  amount  of  phosphorus)  they  contain.  Thus  the 
nuclei  of  spermatozoa,  known  to  consist  of  nearly  pure  nucleic  acid, 
stain  most  intensely  with  basic  dyes,  those  of  ordinary  tissue-cells, 
which  contain  less  phosphorus,  less  intensely.  Malfatti  {'91)  tested 
various  members  of  the  nuclein-series,  vSynthetically  produced  as 
combinations  of  egg-albumin  and  nucleic  acid  from  yeast,  with  a 
mixture  of  red  acid  fuchsin  and  basic  methyl-green.  With  this 
combination  free  nucleic  acid  was  coloured  pure  green,  nucleins 
containing  less  phosphorus  became  bluish-violet,  those  with  little 
or  no  phosphorus  pure  red.  Lilienfeld' s  more  precise  experiments 
in  this  direction  ('92,  '93)  led  to  similar  results.  His  starting-point 
was  given  by  the  results  of  Kossel's  researches  on  the  relations  of 
the  nuclein  group,  which  are  expressed  as  follows :  ^  — 

1  It  has  long  been  known  that  a  form  of  "  nuclein "  may  also  be  obtained  from  the 
nucleo-albumins  of  the  cytoplasm,  e.g.  from  the  yolk  of  hens'  eggs  (vitellin).  Such  nu- 
cleins differ,  however,  from  those  of  nuclear  origin  in  not  yielding  as  cleavage-products  the 
nuclein  bases  (adenin,  xanthin,  etc.).  The  term  "  paranuclein  "  (Kossel)  or  "  pseudo-nuclein  " 
(Hammarsten)  has  therefore  been  suggested  for  this  substance.  True  nucleins  containing 
a  large  percentage  of  albumin  are  distinguished  as  nucleo-proteids.  They  may  be  split  into 
albumin  and  nucleic  acid,  the  latter  yielding  as  cleavage-products  the  nuclein  bases.  Pseudo- 
nucleins  containing  a  large  percentage  of  albumin  are  designated  as  Jiucleo-albumins,  which 
in  like  manner  split  into  albumin  and  paranucleic  or  pseudo-nucleic  acid,  which  yields  no 
nuclein  bases.      (See  Hammarsten,  '94.) 

2  See  Ehrlich,  '79. 

^  From  Lilienfeld,  after  Kossel,  '92,  p.  129. 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM       243 

Niicleo-albumin  (i  %  of  P  or  less), 
by  peptic  digestion  splits  into 


Peptone  Nnclein  (3-4  %  P), 

/  by  treatment  with  acids  splits  into 

Albumin  Nucleic  acid  (9-10  %  P), 

heated  with  mineral  acids  splits  into 

Phosphoric  acid  Nuclein  bases  (A  carbohydrate.') 

(adenin,  guanin,  etc.). 

Now,  according  to  Kossel  and  Lilienfeld,  the  principal  nucleo- 
albumin  (nucleo-proteid)  in  the  nucleus  of  leucocytes  is  micleo-Jiistony 
containing  about  3  %  of  phosphorus,  which  may  be  split  into  a  form 
of  nuclem  playing  the  part  of  an  acid,  and  an  albuminoid  base,  the 
histon  of  Kossel ;  the  nuclein  may  in  turn  be  split  into  albumin  and 
nucleic  acid.  These  four  substances  —  albumin,  nucleo-histon,  nu- 
clein, nucleic  acid  —  thus  form  a  series  in  which  the  proportion  of 
phosphorus,  i.e.  of  nucleic  acid,  successively  increases  from  zero  to 
9-10  %.  If  the  members  of  this  series  be  treated  with  the  same 
mixture  of  red  acid  fuchsin  and  basic  methyl-green,  the  result  is 
as  follows.  Albumin  (egg-albumin)  is  stained  red,  nucleo-histon 
greenish-blue,  nuclein  bluish-green,  nucleic  acid  intense  green.  *'We 
see,  therefore,  that  the  principle  that  determines  the  staining  of  the 
nuclear  substances  is  always  the  nucleic  acid.  All  the  nuclear  sub- 
stances, from  those  richest  in  albumin  to  those  poorest  in  it,  or  con- 
taining none,  assume  the  tone  of  the  nuclear  {i.e.  basic)  stain,  but 
the  combined  albumin  modifies  the  green  more  or  less  towards  blue."  ^ 
Lilienfeld  explains  the  fact  that  chromatin  in  the  cell-nucleus  seldom 
appears  pure  green  on  the  assumption,  supported  by  many  facts, 
that  the  proportion  of  nucleic  acid  and  albumin  vary  with  different 
physiological  conditions,  and  he  suggests  further  that  the  intense 
staining-power  of  the  chromosomes  during  mitosis  is  probably  due 
to  the  fact  that  they  consist,  like  the  chromatin  of  spermatozoa, 
of  pure  or  nearly  pure  nucleic  acid.  Very  interesting  and  con- 
vincing is  a  comparison  of  the  foregoing  staining-reactions  with 
those  given  by  a  mixture  of  a  I'ed  basic  dye  (saffranin)  and  a  green 
acid  one  (''light  green").  With  this  combination  an  effect  is 
given  which  reverses  that  of  the  Biondi-Ehrlich  mixture  ;  i.e.  the 
nuclein  is  coloured  red,  the  albumin  green.  This  is  a  beautiful 
demonstration  of  the  fact  that  staining-reagents  cannot  be  logically 
classified  according  to  colour,  but  only  according  to  their  chemical 

^  I.e.,  p.  394. 


244      SOME   ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

nature.  Such  terms  as  "erythrophilous,"  **cyanophilous,"  and  the 
like  have  therefore  no  meaning  apart  from  the  chemical  compo- 
sition both  of  the  dye  and  of  the  substance  stained.^ 

The  constancy  and  accuracy  of  these  reactions  await  further  test, 
and  until  this  has  been  carried  out  we  should  be  careful  not  to  place 
too  implicit  a  trust  in  the  staining-reactions  as  an  indication  of  chemi- 
cal nature,  especially  as  they  are  known  to  be  affected  by  the  pre- 
ceding mode  of  fixation.  They  afford,  nevertheless,  a  rough  method 
for  the  micro-chemical  test  of  the  proportion  of  nucleic  acid  present 
in  the  nuclear  structures,  and  this  in  the  hands  of  Heidenhain  has 
led  to  some  suggestive  results.  Leucocytes  stained  with  the  Biondi- 
Ehrlich  mixture  of  acid  fuchsin  and  methyl-green  show  the  following 
reactions.  Cytoplasm,  centrosome,  attraction-sphere,  astral  rays,  and 
spindle-fibres  are  stained  pure  red.  The  nuclear  substance  shows  a 
very  sharp  differentiation.  The  chromatic  network  and  the  chromo- 
somes of  the  mitotic  figure  are  green.  The  linin-substance  and  the 
true  nucleoli  or  plasmosomes  appear  red,  like  the  cytoplasm. 
The  linin-network  of  leucocytes  is  stated  by  Heidenhain  to  consist 
of  two  elements,  namely,  of  red  granules  or  microsomes  sus- 
pended in  a  colourless  network.  The  latter  alone  is  called  "  linin  " 
by  Heidenhain.  To  the  red  granules  is  applied  the  term  "oxychro- 
matin,"  while  the  green  substance  of  the  ordinary  chromatic  network, 
forming  the  "  chromatin  "  of  Flemming,  is  called  "  basichromatin."  ^ 
Morphologically,  the  granules  of  both  kinds  are  exactly  alike,^  and 
in  many  cases  the  oxychromatin-granules  are  found  not  only  in 
the  "  achromatic  "  nuclear  network,  but  also  intermingled  with  the 
basichromatin-granules  of  the  chromatic  network.  Collating  these 
results  with  those  of  the  physiological  chemists,  Heidenhain  concludes 
that  basichromatin  is  a  substance  rich  in  phosphorus  {i.e.  nucleic 
acid),  oxychromatin  a  substance  poor  in  phosphorus,  and  that, 
further,  "  basichromatin  and  oxychromatin  are  by  no  means  to  be 
regarded  as  permanent  unchangeable  bodies,  but  may  change  their 
colour-reactions  by  combining  with  or  giving  off  phosphorus."  In 
other  words,  "the  affinity  of  the  chromatophilous  microsomes  of  the 
nuclear  network  for  basic  and  acid  aniline  dyes  are  regulated  by  cer- 
tain physiological  conditions  of  the  nucleus  or  of  the  cell."* 

This  conclusion,  which  is  entirely  in  harmony  with  the  statements 
of  Kossel  and  Halliburton  quoted  above,  opens  up  the  most  interest- 
ing questions  regarding  the  periodic  changes  in  the  nucleus.  The 
staining-power  of  chromatin  is  at  a  maximum  when  in  the  preparatory 
stages  of  mitosis  (spireme-thread,  chromosomes).  During  the  ensuing 
growth  of  the  nucleus  it  always  diminishes,  suggesting  that  a  com- 

1  Cf.  p.  127.  2  '9^^  p.  543.  8 1^,^^  p^  ^47.  4  ic.,  p.  548. 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM 


245 


bination  with  albumin  has  taken  place.  This  is  illustrated  in  a  very 
striking  way  by  the  history  of  the  egg-nucleus  or  germinal  vesicle, 
which  exhibits  the  nuclear  changes  on  a  large  scale.  It  has 
long  been  known  that  the  chromatin  of  this  nucleus  undergoes 
great  changes  during  the  growth  of  the  ^gg,  and  several  observers 
have  maintained  its  entire  disappearance  at  one  period.  Riickert 
first  carefully  traced  out  the  history  of  the  chromatin  in  detail  in  the 


Fig.  no. —  Chromosomes  of  ihe  germinal  vesicle  in  the  shark  Pristiurus,  at  different  periods, 
drawn  to  the  same  scale.     [RiJCKERT.] 

A.  At  the  period  of  maximal  size  and  minimal  staining-capacity  (egg  3  mm.  in  diameter). 
D.  Later  period  (egg  13  mm.  in  diameter).  C.  At  the  close  of  ovarian  life,  of  minimal  size  and 
maximal  staining-power. 


eggs  of  sharks,  and  his  general  results  have  since  been  confirmed  by 
Born  in  the  eggs  of  Triton,  In  the  shark  Pristiiuiis  Riickert  ('92,  i) 
finds  that  the  chromosomes,  which  persist  throughout  the  entire 
growth-period  of  the  Q,gg,  undergo  the  following  changes  (Fig.  1 10) : 
At  a  very  early  stage  they  arc  small,  and  stain  intensely  with  nuclear 
dyes.  During  the  growth  of  the  ^g'g  they  undergo  a  great  increase 
in  size,  and  progressively  lose  their  staining-capacity.  At  the  same 
time  their  surface  is  enormously  increased  by  the  development  of 
long  threads  which  grow  out  in  every  direction  from  the  central  axis 


246      SOME   ASPECTS    OF  CELL-CHEMISTRY  AND    CELL-PIIYSIOLOGY 

(Fig.  1 10,  A).  As  the  ^^g  approaches  its  full  size,  the  chromosomes 
rapidly  diminish  in  size,-  the  radiating  threads  disappear,  and  the  stain- 
ing-capacity  increases  (Fig.  1 10,  ^).  They  are  finally  again  reduced  to 
minute  intensely  staining  bodies  which  enter  into  the  equatorial  plate 
of  the  first  polar  mitotic  figure  (Fig.  1 10,  C).  How  great  the  change 
of  volume  is  may  be  seen  from  the  following  figures.  At  the  beginning 
the  chromosomes  measure,  at  most,  12  /a  (about  2  oVo"  ^'^•)  ^^  length  and 
|-/x  in  diameter.  At  the  height  of  their  development  they  are  almost 
eight  times  their  original  length  and  twenty  times  their  original 
diameter.  In  the  final  period  they  are  but  2  /x  in  length  and  i  /x  in  di- 
ameter. These  measurements  show  a  change  of  volume  so  enormous, 
even  after  making  due  allowance  for  the  loose  structure  of  the  large 
chromosomes,  that  it  cannot  be  accounted  for  by  mere  swelling  or 
shrinkage.  The  chromosomes  evidently  absorb  a  large  amount  of 
matter,  combine  with  it  to  form  a  substance  of  diminished  staining- 
'capacity,  and  finally  give  off  matter,  leaving  an  intensely  staining 
substance  behind.  As  Riickert  points  out,  the  great  increase  of  sur- 
face in  the  chromosomes  is  adapted  to  facilitate  an  exchange  of  mate- 
rial between  the  chromatin  and  the  surrounding  substance;  and  he 
concludes  that  the  coincidence  between  the  growth  of  the  chromo- 
somes and  that  of  the  ^gg,  points  to  an  intimate  connection  between 
the  nuclear  activity  and  the  formative  energy  of  the  cytoplasm. 

If  these  facts  are  considered  in  the  light  of  the  known  stain- 
ing-reaction  of  the  nuclein  series,  we  must  admit  that  the  follow- 
ing conclusions  are  something  more  than  mere  possibilities.  We 
may  infer  that  the  original  chromosomes  contain  a  high  percent- 
age of  nucleic  acid ;  that  their  growth  and  loss  of  staining-power  is 
due  to  a  combination  with  a  large  amount  of  albuminous  substance 
to  form  a  lower  member  of  the  nuclein  series,  perhaps  even  a  nucleo- 
albumin ;  that  their  final  diminution  in  size  and  resumption  of  staining- 
power  is  caused  by  a  giving  up  of  the  albumin  constituent,  restoring 
the  nuclein  to  its  original  state  as  a  preparation  for  division.  The 
growth  and  diminished  staining-capacity  of  the  chromatin  occurs 
during  a  period  of  intense  constructive  activity  in  the  cytoplasm ;  its 
diminution  in  bulk  and  resumption  of  staining-capacity  coincides  with 
the  cessation  of  this  activity.  This  result  is  in  harmony  with  the 
observations  of  Schwarz  and  Zacharias  on  growing  plant-cells,  the 
percentage  of  nuclein  in  the  nuclei  of  embryonic  cells  (meristem) 
being  at  first  relatively  large  and  diminishing  as  the  cells  increase  in 
size.  It  agrees  further  with  the  fact  that  of  all  forms  of  nuclei  those 
of  the  spermatozoa,  in  which  growth  is  suspended,  are  richest  in 
nucleic  acid,  and  in  this  respect  stand  at  the  opposite  extreme  from 
the  nuclei  of  the  rapidly  growing  egg-cell. 

Accurately  determined  facts  in  this  direction  are  still  too  scanty  to 


CHEMICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM       247 

admit  of  a  safe  generalization.  They  arc,  however,  enough  to  indi- 
cate the  probabiUty  that  chromatin  may  pass  through  a  certain  cycle 
in  the  life  of  the  cell,  the  percentage  of  albumin  increasing  during 
the  vegetative  activity  of  the  nucleus,  decreasing  in  its  reproductive 
phase.  In  other  words,  a  combination  of  albumin  with  nuclein  or 
nucleic  acid  is  an  accompaniment  of  constructive  metabolism.  As 
the  cell  prepares  for  division,  the  combination  is  dissolved  and  the 
nuclein-radicle  or  nucleic  acid  is  handed  on  by  division  to  the  daugh- 
ter-cells. It  is  a  tempting  hypothesis,  suggested  to  me  by  Mr.  A.  P. 
Mathews  on  the  basis  of  Kossel's  work,  that  the  nuclein  is  in  a  chem- 
ical sense  the  formative  centre  of  the  cell,  attracting  to  it  the  food- 
matters,  entering  into  loose  combination  with  them,  and  giving  them 
off  to  the  cytoplasm  in  an  elaborated  form.  Could  this  be  estab- 
lished, we  should  have  a  clue  to  the  nuclear  control  of  the  cell 
through  the  process  of  synthetic  metaboHsm.  Claude  Bernard 
advanced  a  nearly  similar  hypothesis  two  score  years  ago  dj^),  main- 
taining that  the  cytoplasm  is  the  seat  of  destructive  metabolism,  the 
nucleus  the  organ  of  constructive  metabolism  and  organic  synthesis, 
and  insisting  that  the  role  of  the  nucleus  in  nutrition  gives  the  key 
to  its  significance  as  the  organ  of  development,  regeneration,  and 
inheritance.^ 

That  the  nucleus  is  especially  concerned  in  synthetic  metabolism 
is  now  becoming  more  and  more  clearly  recognized  by  physiological 
chemists.  Kossel  concludes  that  the  formation  of  new  organic  matter 
is  dependent  on  the  nucleus,^  and  that  nuclein  in  some  manner  plays 
a  leading  role  in  this  process ;  and  he  makes  some  interesting  sugges- 
tions regarding  the  synthesis  of  complex  organic  matters  in  the  living 
cell  with  nuclein  as  a  starting-point.  Chittenden,  too,  in  a  review  of 
recent  chemico-physiological  discoveries  regarding  the  cell,  concludes  : 
''The  cell-nucleus  may  be  looked  upon  as  in  some  manner  standing  in 
close  relation  to  those  processes  which  have  to  do  wdth  the  formation 
of  organic  substances.  Whatever  other  functions  it  may  possess,  it 
evidently,  through  the  inherent  qualities  of  the  bodies  entering  into 
its  composition,  has  a  controlling  power  over  the  metabolic  processes 
in  the  cell,  modifying  and  regulating  the  nutritional  changes"  ('94). 

1  '*  II  semble  done  que  la  eellule  qui  a  perdu  son  noyau  soit  sterilisee  au  point  de  vue  de 
la  generation,  c'est  a  dire  de  la  synthese  morphologique,  et  qu'elle  le  soit  aussi  au  point  de 
vue  de  la  synthese  chimique,  car  elle  cesse  de  produire  des  principes  immediats,  et  ne  peut 
guere  qu'oxydcr  et  detruire  ceux  qui  s'y  etaient  accumules  par  une  elaboration  anterieure  du 
noyau.  II  semble  done  que  le  noyau  soit  Xo.  germe  de  nutrition  de  la  cellule;  il  attire  autour 
de  lui  et  elabore  les  materiaux  nutritifs  "  ('78,  p,  523). 

■^  Schiefferdecker  und  Kossel,  Gewebelehre^  p.  57. 


248       SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 


B.     Physiological  Relations  of  Nucleus  and  Cytoplasm 

How  nearly  the  foregoing  facts  bear  on  the  problem  of  the  form- 
ative power  of  the  cell  in  a  morphological  sense  is  obvious,  and  they 
have  in  a  measure  anticipated  certain  conclusions  regarding  the  ivle  of 
nucleus  and  cytoplasm  which  we  may  now  examine  from  a  somewhat 
different  point  of  view. 

Briicke  long  ago  drew  a  clear  distinction  between  the  chemical  and 
molecular  composition  of  organic  substances,  on  the  one  hand,  and, 
on  the  other  hand,  their  definite  grouping  in  the  cell  by  which  arises 
organization  in  a  morphological  sense.  Claude  Bernard,  in  like  man- 
ner, distinguished  between  chemical  synthesis,  through  which  organic 
matters  are  formed,  and  morphological  synthesis,  by  which  they  are 
built  into  a  specifically  organized  fabric  ;  but  he  insisted  that  these  two 
processes  are  but  different  phases  or  degrees  of  the  same  phenome- 
non, and  that  both  are  expressions  of  the  nuclear  activity.  We  have 
now  to  consider  some  of  the  evidence  that  the  formative  power  of  the 
cell,  in  a  morphological  sense,  centres  in  the  nucleus,  and  that  this  is 
therefore  to  be  regarded  as  the  especial  organ  of  inheritance.  This 
evidence  is  mainly  derived  from  the  comparison  of  nucleated  and 
non-nucleated  masses  of  protoplasm  ;  from  the  form,  position  and 
movements  of  the  nucleus  in  actively  growing  or  metabolizing  cells ; 
and  from  the  history  of  the  nucleus  in  mitotic  cell-division,  in  fer- 
tilization, and  in  maturation. 

I.    Experiine^its  on   Unicellular  Organisms 

Brandt  ^7J^  long  since  observed  that  enucleated  fragments  of 
ActinosphcBrinm  soon  die,  while  nucleated  fragments  heal  their  wounds 
and  continue  to  live.  The  first  decisive  comparison  between  nucle- 
ated and  non-nucleated  masses  of  protoplasm  was,  however,  made  by 
Moritz  Nussbaum  in  1884  in  the  case  of  an  infusorian,  Oxytricha. 
If  one  of  these  animals  be  cut  into  two  pieces,  the  subsequent 
behaviour  of  the  two  fragments  depends  on  the  presence  or  absence 
of  the  nucleus  or  a  nuclear  fragment.  The  nucleated  fragments 
quickly  heal  the  wound,  regenerate  the  missing  portions,  and  thus 
produce  a  perfect  animal.  On  the  other  hand,  enucleated  fragments, 
consisting  of  cytoplasm  only,  quickly  perish.  Nussbaum  therefore 
drew  the  conclusion  that  the  nucleus  is  indispensable  for  the  forma- 
tive energy  of  the  cell.  The  experiment  was  soon  after  repeated  by 
Gruber  ('85)  in  the  case  of  Stentor,  another  infusorian,  and  with  the 
same  result  (Fig.  1 12).  Fragments  possessing  a  large  fragment  of  the 
nucleus  completely  regenerated  within  twenty-four  hours.     If  the  nu- 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM    249 


clear  fragment  were  smaller,  the  regeneration  proceeded  more  slowly. 
If  no  nuclear  substance  were  present,  no  regeneration  took  place, 
though  the  wound  closed  and  the  fragment  lived  for  a  considerable 
time.  The  only  exception  —  but  it  is  a  very  significant  one  —  was  the 
case  of  individuals  in  which  the  process  of  normal  fission  had  begun ; 
in  these  a  non-nucleated  fragment  in  which  the  formation  of  a  new 
peristome  had  already  been  initiated  healed  the  wound  and  com- 
pleted the  formation  of  the  peristome.  Lillie  ('96)  has  recently 
found  that  Stentor  may  by 
shaking  be  broken  into  frag- 
ments of  all  sizes,  and  that 
nucleated  fraefments  as  small 


as 


2  Y  the  volume  of  the  entire 
animal    are    still 


are  still  capable  of 
complete  regeneration.  All 
non-nucleated  fragments  per- 
ish. 

These  studies  of  Nussbaum 
and  Gruber  formed  a  prelude 
to  more  extended  investiga- 
tions in  the  same  direction 
by  Gruber,  Balbiani,  Hofer, 
and  especially  Verworn. 
Verworn  i^^'^)  proved  that 
in  Polystoinclla,  one  of  the 
Foraminifera,  nucleated  frag- 
ments are  able  to  repair  the 
shell,  while  non-nucleated 
fragments  lack  this  power. 
Balbiani  ('89)  showed  that 
although  non-nucleated  frag- 
ments of  infusoria  had  no 
power  of  regeneration,  they  might  nevertheless  continue  to  live  and 
swim  actively  about  for  many  days  after  the  operation,  the  con- 
tractile vacuole  pulsating  as  usual.  Hofer  ('89),  experimenting  on 
Avtceba,  found  that  non-nucleated  fragments  might  live  as  long 
as  fourteen  days  after  the  operation  (Fig.  113).  Their  movements 
continued,  but  were  somewhat  modified,  and  little  by  little  ceased,  but 
the  pulsations  of  the  contractile  vacuole  were  but  slightly  affected ; 
they  lost  more  or  less  completely  the  capacity  to  digest  food,  and 
the  power  of  adhering  to  the  substratum.  Nearly  at  the  same  time 
Verworn  ('89)  published  the  results  of  an  extended  comparative 
investigation  of  various  Protozoa  that  placed  the  whole  matter  in 
a  very    clear    light.      His    experiments,   while   fully    confirming   the 


Fig.  III. —  Stylonychia,  and  enucleated  frag- 
ments,    [Verworn.] 

At  the  left  an  entire  animal,  showing  planes  of 
section.  The  middle-piece,  containing  two  nuclei, 
regenerates  a  perfect  animal.  The  enucleated  pieces, 
shown  at  the  right,  swim  about  for  a  time,  but  finally 
perish. 


2 so      SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

accounts  of  his  predecessors  in  regard  to  regeneration,  added  many 
extremely  important  and  significant  results.  Non-nucleated  frag- 
ments both  of  infusoria  {e.g.,  LacJirymaria)  and  rhizopods  {Poly- 
stomella,  TJialassicolld)  not  only  live  for  a  considerable  period,  but 
perform  perfectly  normal  and  characteristic  movements,  show  the 
same  susceptibility  to  stimulus,  and  have  the  same  power  of  ingulf- 
ing food,  as  the  nucleated  fragments.  They  lack,  Jiowever,  the  power 
of  digestion  and  secretion.     Ingested  food-matters   may  be    slightly 


Fig.  112.  —  Regeneration  in  the  unicellular  animal  Stentor.      [Grui?ER.] 
A.  Animal  divided  into  three  pieces,  each  containing  a  fragment  of  the  nucleus.     B.  The 
three  fragments  shortly  afterwards.     C,  The  three  fragments  after  twenty-four  hours,  each  regen- 
erated to  a  perfect  animal. 


altered,  but  are  never  completely  digested.  The  non-nucleated  frag- 
ments are  unable  to  secrete  the  material  for  a  new  shell  {Polysto- 
mella)  or  the  slime  by  which  the  animals  adhere  to  the  substratum 
{Amoeba,  Diffliigia,  Polystomclla).  Beside  these  results  should  be 
placed  the  well-known  fact  that  dissevered  nerve-fibres  in  the 
higher  animals  are  only  regenerated  from  that  end  which  remains 
in  connection  with  the  nerve-cell,  while  the  remaining  portion  inva- 
riably degenerates. 

These  beautiful  observations  prove  that  destructive  metabolism,  as 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM    25 1 

manifested  by  co-ordinated  forms  of  protoplasmic  contractility,  may 
go  on  for  some  time  undisturbed  in  a  mass  of  cytoplasm  deprived  of 
a  nucleus.  On  the  other  hand,  the  formation  of  new  chemical  or 
morphological  products  by  the  cytoplasm  only  takes  place  in  the  pres- 
ence of  a  nucleus.  These  facts  form  a  complete  demonstration  that 
the  nucleus  plays  an  essential  part  not  only  in  the  operations  of  syn- 
thetic metabolism  or  chemical  synthesis,  but  also  in  the  morphological 


Fig.  113.  —  Nucleated  and  non-nucleated  fragments  of  Amceba.     [HOFER.] 
A.  D.  An  Amceba  divided  into  nucleated  and  non-nucleated  halves,  five  minutes  after  the  opera- 
tion.    C.  D.  The  two  halves  after  eight  days,  each  containing  a  contractile  vacuole. 


dctciinination  of  these  operatiojis,  i.e.  the  morphological  synthesis  of 
Bernard  —  a  point  of  capital  importance  for  the  theory  of  inheritance, 
as  will  appear  beyond. 

Convincing  experiments  of  the  same  character  and  leading  to  the 
same  result  have  been  made  on  the  unicellular  plants.  Klebs 
observed  as  long  ago  as  1879  that  naked  protoplasmic  fragments  of 
VaucJieria  and  other  algae  were  incapable  of  forming  a  new  cellulose 
membrane  if  devoid  of  a  nucleus ;    and  he  afterwards  showed  ('87) 


2  52      SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

that  the  same  is  true  of  Zygtieina  and  CEdigoniuvi.  By  plasmolysis 
the  cells  of  these  forms  may  be  broken  up  into  fragments,  both 
nucleated  and  non-nucleated.  The  former  surround  themselves  with 
a  new  wall,  grow,  and  develop  into  complete  plants ;  the  latter,  while 
able  to  form  starch  by  means  of  the  chlorophyll  they  contain,  are 
incapable  of  utilizing  it,  and  are  devoid  of  the  power  of  forming  a 
new  membrane,  and  of  growth  and  regeneration. ^ 

Although  Verworn's  results  confirm  and  extend  the  earlier  work  of 
Nussbaum  and  Gruber,  he  has  drawn  from  them  a  somewhat  different 
conclusion,  based  mainly  on  the  fact,  determined  by  him,  that  a 
nucleus  deprived  of  cytoplasm  is  as  devoid  of  the  power  to  regenerate  ' 
the  whole  as  an  enucleated  mass  of  cytoplasm.  From  this  he  argues, 
with  perfect  justice,  that  the  formative  energy  cannot  properly  be 
ascribed  to  the  nucleus  alone,  but  is  rather  a  co-ordinate  activity  of 
both  nucleus  and  cytoplasm.  No  one  will  dispute  this  conclusion; 
yet  in  the  light  of  other  evidence  it  is,  I  think,  stated  in  somewhat 
misleading  terms  which  obscure  the  significance  of  Verworn's  own 
beautiful  experiments.  It  is  undoubtedly  true  that  the  cell,  like  any 
other  living  organism,  acts  as  a  whole,  and  that  the  integrity  of  all  of 
its  parts  is  necessary  to  its  continued  existence ;  but  this  no  more  pre- 
cludes a  specialization  and  localization  of  function  in  the  cell  than  in 
the  higher  organism.  The  experiments  certainly  do  not  prove  that 
the  nucleus  is  the  sole  instrument  of  organic  synthesis,  but  they  no 
less  certainly  indicate  its  especial  importance  in  this  process.  The 
sperm-nucleus  is  unable  to  develop  its  latent  capacities  without  be- 
coming associated  with  the  cytoplasm  of  an  ovum,  but  its  significance 
as  the  bearer  of  the  paternal  heritage  is  not  thereby  lessened  one  iota. 

2.    Position  and  Movements  of  the  Nnclens 

Many  observers  have  approached  the  same  problem  from  a  dif- 
ferent direction  by  considering  the  position,  movements,  and  changes 
of  form  in  the  nucleus  with  regard  to  the  formative  activities  in 
the  cytoplasm.  To  review  these  researches  in  full  would  be  impos- 
sible, and  we  must  be  content  to  consider  only  the  well-known 
researches  of  Haberlandt  (^Jj)  and  Korschelt  ('89),  both  of  whom 
have  given  extensive  reviews  of  the  entire  subject  in  this  regard. 
Haberlandt's  studies  related  to  the  position  of  the  nucleus  in  plant- 
cells  with  especial  regard  to  the  growth  of  the  cellulose  membrane. 
He  determined  the  very  significant  fact  that  local  growth  of  the 
cell-wall  is  always  preceded  by  a  movement  of  the  nucleus  to    the 

1  Palla  ('90)  has  disputed  this  result,  maintaining  that  enucleated  masses  of  protoplasm 
pressed  out  from  pollen-tubes  might  surround  themselves  with  membranes  and  grow  out 
into  long  tubes.  Later  observations,  however,  by  Acqua  ('91),  throw  doubt  on  Palla's 
conclusion. 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM    253 

point  of  growth.  Thus,  in  the  formation  of  epidermal  cells  the 
nucleus  lies  at  first  near  the  centre,  but  as  the  outer  wall  thickens, 
the  nucleus  moves  towards  it,  and  remains  closely  applied  to  it 
throughout  its  growth,  after  which  the  nucleus  often  moves  into 
another  part  of  the  cell  (Fig.  114,  A^  B).  That  this  is  not  due 
simply  to  a  movement  of  the  nucleus  towards  the  air  and  light  is 
beautifully  shown  in  the  coats  of  certain  seeds,  where  the  nucleus 


'/i 


*  ■ 


Fig.  114.  —  Position  of  the  nuclei  in  growing  plant-cells.     [Haberlandt.] 
A.  Young  epidermal  cell  of  Luzula  with  central  nucleus,  before  thickening  of  the  membrane. 
B.  Three  epidermal  cells  of  Monstera,  during  the  thickening  of  the  outer  wall.     C.  Cell  from  the 
seed-coat  of  Scopulina  during  the  thickening  of  the  inner  wall.     D.  E.  Position  of  the  nuclei  dur- 
ing the  formation  of  branches  in  the  root-hairs  of  the  pea. 


moves  not  to  the  outer,  but  to  the  inner  wall  of  the  cell,  and  here 
the  thickening  takes  place  (Fig.  114,  C).  The  same  position  of  the 
nucleus  is  shown  in  the  thickening  of  the  walls  of  the  guard-cells 
of  stomata,  in  the  formation  of  the  peristome  of  mosses,  and  in 
many  other  cases.  In  the  formation  of  root-hairs  in  the  pea,  the 
primary  outgrowth  always  takes  place  from  the  immediate  neighbour- 
hood of  the  nucleus,  which  is  carried  outward  and  remains  near  the 
tip  of  the  growing  hair  (Fig.    1 14,  D,  E).     The  same  is  true  of  the 


254      SOME   ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

rhizoids  of  fern-prothallia  and  liverworts.  In  the  hairs  of  aerial 
plants  this  rule  is  reversed,  the  nucleus  lying  near  the  base  of  the 
hair;  but  this  apparent  exception  proves  the  rule,  for  both  Hunter 
and  Haberlandt  show  that  in  this  case  growth  of  the  hair  is  not 
apical,  but  proceeds  from  the  base  !  Very  interesting  is  Haberlandt's 
observation  that  in  the  regeneration  of  fragments  of  Vanchcria  the 
growing  region,  where  a  new  membrane  is  formed,  contains  no 
chlorophyll,  but  numerous  nuclei.  The  general  result,  based  on  the 
study  of  a  large  number  of  cases,  is  in  Haberlandt's  words  that 
*'the  nucleus  is  in  most  cases  placed  in  the  neighbourhood,  more  or 
less  immediate,  of  the  points  at  which  growth  is  most  active  and 
continues  longest."  This  fact  points  to  the  conclusion  that  'Mts 
function  is  especially  connected  with  the  developmental  processes 
of  the  cell,"  ^  and  that  "  in  the  growth  of  the  cell,  more  especially 
in  the  growth  of  the  cell-wall,  the  nucleus  plays  a  definite  part." 
Korschelt's  work  deals  especially  with  the  correlation  between 
form  and  position  of  the  nucleus  and  the  nutrition  of  the  cell; 
and  since  it  bears  more  directly  on  chemical  than  on  morphologi- 
cal synthesis,  may  be  only  briefly  reviewed  at  this  point.  His 
general  conclusion  is  that  there  is  a  definite  correlation,  on  the 
one  hand  between  the  position  of  the  nucleus  and  the  source  of 
food-supply,  on  the  other  hand  between  the  size  of  the  nucleus 
and  the  extent  of  its  surface  and  the  elaboration  of  material  by 
the  cell.  In  support  of  the  latter  conclusion  many  cases  are  brought 
forward  of  secreting  cells  in  which  the  nucleus  is  of  enormous  size 
and  has  a  complex  branching  form.  Such  nuclei  occur,  for  example, 
in  the  silk-glands  of  various  lepidopterous  larvae  (Meckel,  Zaddach, 
etc.),  which  are  characterized  by  an  intense  secretory  activity  con- 
centrated into  a  very  short  period.  Here  the  nucleus  forms  a 
labyrinthine  network  (Fig.  ii,  if),  by  which  its  surface  is  brought  to 
a  maximum,  pointing  to  an  active  exchange  of  material  between 
nucleus  and  cytoplasm.  The  same  type  of  nucleus  occurs  in  the 
Malpighian* tubules  of  insects  (Leydig,  R.  Hertwig),  in  the  spinning- 
glands  of  amphipods  (Mayer),  and  especially  in  the  nutritive  cells 
of  the  insect  ovary  already  referred  to  at  p.  114.  Here  the  develop- 
ing ovum  is  accompanied  and  surrounded  by  cells,  which  there  is 
good  reason  to  believe  are  concerned  with  the  elaboration  of  food 
for  the  egg-cell.  In  the  earwig  Forficida  each  ^gg  is  accompanied 
by  a  single  large  nutritive  cell  (Fig.  115),  which  has  a  very  large 
nucleus  rich  in  chromatin  (Korschelt).  This  cell  increases  in  size 
as  the  ovum  grows,  and  its  nucleus  assumes  the  complex  branching 
form  shown  in  the  figure.     In  the  butterfly  Vanessa  there  is  a  group 

1  i.e.,  p.  99. 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND    CYTOPLASM    255 


of  such  cells  at  one  pole  of  the  ^gg  from  which  the  latter  is  believed 
to  draw  its  nutriment  (Fig.  58).  A  very  interesting  case  is  that  of 
the  annelid  Ophryotrocha,  referred  to  at  p.  114.  Here,  as  described 
by  Korschelt,  the  Qgg  floats  in  the  perivisceral  fluid,  accompanied 
by  a  nursfe-cell  having  a  very  large  chromatic  nucleus,  while  that  of 
the  Qgg  is  smaller  and 
poorer  in  chromatin. 
As  the  Qgg  completes 
its  growth,  the  nurse- 
cell  dwindles  away  and 
finally  perishes  (Fig.  57). 
In  all  these  cases  it 
is  scarcely  possible  to 
doubt  that  the  Qgg  is 
in  a  measure  relieved 
of  the  task  of  elaborat- 
ing cytoplasmic  products 
by  the  nurse-cell,  and 
that  the  great  develop- 
ment of  the  nucleus  in 
the  latter  is  correlated 
with  this  function. 

Regarding  the  posi- 
tion and  movements  of 
the  nucleus,  Korschelt 
reviews  many  facts 
pointing  towards  the 
same  conclusion.  Per- 
haps the  most  sugges- 
tive of  these  relate  to 
the  nucleus  of  the  Qgg 
during  its  ovarian  his- 
tory. In  many  of  the 
insects,  as  in  both  the 
cases  referred  to  above, 
the  egg-nucleus  at  first  occupies  a  central  position,  but  as  the 
Qgg  begins  to  grow,  it  moves  to  the  periphery  on  the  side  turned 
towards  the  nutritive  cells.  The  same  is  true  in  the  ovarian 
eggs  of  some  other  animals,  good  examples  of  which  are  afforded  by 
various  coelenterates,  eg,  in  medusae  (Claus,  Hertwig)  and  actinians 
(Korschelt,  Hertwig),  where  the  germinal  vesicle  is  always  near  the 
point  of  attachment  of  the  Qgg.  Most  suggestive  of  all  is  the  case 
of  the  water-beetle  Dytiscus,  in  which  Korschelt  was  able  to  observe 
the  movements  and  changes  of  form  in  the  living  object.     The  eggs 


b 


r^^  :^  a 


-o^^4;' 


Fig.  115.  —  Upper  portion  of  the  ovary  in  the  earwig 
Forjicula,  showing  eggs  and  nurse-cells.     [KORSCHELT.] 

Below,  a  portion  of  the  nearly  ripe  ^g'g  (^),  showing  deuto- 
plasm-spheres  and  germinal  vesicle  {gv).  Above  it  lies  the 
niirse-cell  [it)  with  its  enormous  branching  nucleus.  Two 
successively  younger  stages  of  egg  and  nurse  are  shown  above. 


256      SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

here  lie  in  a  single  series  alternating  with  chambers  of  nutritive  cells. 
The  latter  contain  granules  which  are  believed  by  Korschelt  to  pass 
into  the  Q,gg,  perhaps  bodily,  perhaps  by  dissolving  and  entering  in  a 
liquid  form.  At  all  events,  the  egg  contains  accumulations  of  similar 
granules,  which  extend  inwards  in  dense  masses  from  the  nutritive 
cells  to  the  germinal  vesicle,  which  they  may  more  or  less  completely 
surround.  The  latter  meanwhile  becomes  amoeboid,  sending  out  long 
pseudopodia,  which  are  always  directed  towards  the  principal  mass  of 
granules  (Fig.  58).  The  granules  could  not  be  traced  into  the  nucleus, 
but  the  latter  grows  rapidly  during  these  changes,  proving  that  mat- 
ter must  be  absorbed  by  it,  probably  in  a  liquid  form.^ 

All  of  these  and  a  large  number  of  other  observations  in  the  same 
direction  lead  to  the  conclusion  that  the  cell-nucleus  plays  an  active 
part  in  nutrition,  and  that  it  is  especially  active  during  its  constructive 
phase.  On  the  whole,  therefore,  the  behaviour  of  the  nucleus  in  this 
regard  is  in  harmony  with  the  result  reached  by  experiment  on  the 
one-celled  forms,  though  it  gives  in  itself  a  far  less  certain  and  con- 
vincing result. 

We  now  turn  to  evidence  which,  though  less  direct  than  the  experi- 
mental proof,  is  scarcely  less  convincing.  This  evidence,  which  has 
been  exhaustively  discussed  by  Hertwig,  Weismann,  and  Strasburger, 
is  drawn  from  the  history  of  the  nucleus  in  mitosis,  fertilization,  and 
maturation.  It  calls  for  only  a  brief  review  here,  since  the  facts  have 
been  fully  described  in  earlier  chapters. 

3.    TJie  Nticleiis  in  Mitosis 

To  Wilhelm  Roux  ('83)  we  owe  the  first  clear  recognition  of 
the  fact  that  the  transformation  of  the  chromatic  substance  dur- 
ing mitotic  division  is  manifestly  designed  to  effect  a  precise  di- 
vision of  all  its  parts,  —  i.e.  a  panmeristic  division  as  opposed  to  a 
mere  mass-division,  —  and  their  definite  distribution  to  the  daughter- 
cells.  "The  essential  operation  of  nuclear  division  is  the  divi- 
sion of  the  mother-granules"  {i.e.  the  individual  chromatin-grains) ; 
"all  the  other  phenomena  are  for  the  purpose  of  transporting  the 
daughter-granules  derived  from  the  division  of  a  mother-granule,  one 
to  the  centre  of  one  of  the  daughter-cells,  the  other  to  the  centre  of 
the  other."  In  this  respect  the  nucleus  stands  in  marked  contrast  to 
the  cytoplasm,  which  undergoes  on  the  whole  a  mass-division,  although 
certain  of  its  elements,  such  as  the  plastids  and  the  centrosome,  may 
separately  divide,  like  the  elements  of  the  nucleus.  From  this  fact 
Roux  argued,  first,  that  different  regions  of  the  nuclear  substance 

1  Some  observers  have  maintained  that  the  nucleus  may  take  in  as  well  as  give  off  solid 
matters.     This  statement  rests,  however,  on  a  very  insecure  foundation. 


PHYSIOLOGICAL   RELATIONS   OF  NUCLEUS  AND   CYTOPLASM    2^ J 

must  represent  different  qualities,  and  second,  that  the  apparatus  of 
mitosis  is  designed  to  distribute  these  qualities,  according  to  a 
definite  law,  to  the  daughter-cells.  The  particular  form  in  which 
Roux  and  Weismann  developed  this  conception  has  now  been  gener- 
ally rejeclfed,  and  in  any  form  it  has  some  serious  difficulties  in  its 
way.  We  cannot  assume  a  precise  localization  of  chromatin-ele- 
ments  in  all  parts  of  the  nucleus ;  for  on  the  one  hand  a  large  part 
of  the  chromatin  may  degenerate  or  be  cast  out  (as  in  the  matu- 
ration of  the  ^g^,  and  on  the  other  hand  in  the  Protozoa  a  small 
fragment  of  the  nucleus  is  able  to  regenerate  the  whole.  Neverthe- 
less, the  essential  fact  remains,  as  Hertwig,  Kolliker,  Strasburger, 
De  Vries,  and  many  others  have  insisted,  that  in  mitotic  cell-division 
the  chromatin  of  the  mother-cell  is  distributed  with  the  most  scrupu- 
lous equality  to  the  nuclei  of  the  daughter-cells,  and  that  in  this 
regard  there  is  a  most  remarkable  contrast  between  nucleus  and 
cytoplasm.  This  holds  true  with  such  wonderful  constancy  through- 
out the  series  of  living  forms,  from  the  lowest  to  the  highest,  that  it 
must  have  a  deep  significance.  And  while  we  are  not  yet  in  a  posi- 
tion to  grasp  its  full  meaning,  this  contrast  points  unmistakably  to 
the  conclusion  that  the  most  essential  material  handed  on  by  the 
mother-cell  to  its  progeny  is  the  chromatin,  and  that  this  substance 
therefore  has  a  special  significance  in  inheritance. 

4.    The  Nucleus  in  Fertilization 

The  foregoing  argument  receives  an  overwhelming  reinforce- 
ment from  the  facts  of  fertilization.  Although  the  ovum  supplies 
nearly  all  the  cytoplasm  for  the  embryonic  body,  and  the  sper- 
matozoon at  most  only  a  trace,  the  latter  is  nevertheless  as  potent 
in  its  effect  on  the  offspring  as  the  former.  On  the  other  hand, 
the  nuclei  contributed  by  the  two  germ-cells,  though  apparently 
different,  become  in  '  the  end  exactly  equivalent  in  every  visible 
respect  —  in  structure,  in  staining-reactions,  and  in  the  number  and 
form  of  the  chromosomes  to  which  each  gives  rise.  But  further- 
more the  substance  of  the  two  germ-nuclei  is  distributed  with  abso- 
lute equality,  certainly  to  the  first  two  cells  of  the  embryo,  and 
probably  to  all  later-formed  cells.  The  latter  conclusion,  which 
long  remained  a  mere  surmise,  has  been  rendered  nearly  a  cer- 
tainty by  the  remarkable  observations  of  Riickert,  Zoja,  and  Hacker, 
described  in  Chapters  IV.  and  VI.  The  conclusion  is  irresistible 
that  the  specific  character  of  the  cell  is  in  the  last  analysis  deter- 
mined by  that  of  the  nucleus,  that  is  by  the  chromatin,  and  that  in 
the  equal  distribution  of  paternal  and  maternal  chromatin  to  all  the 
cells  of  the  offspring  we  find  the  physiological  explanation  of  the 


258      SOME  ASPECTS   OF  CELL-CIIEMISTKY  AND    CELL-PHYSIOLOGY 


fact  that  every  part  of  the   latter   may  show  the  characteristics  of 
either  or  both  parents. 

Boveri  ('89,  '95,  i)  has  attempted  to  test  this  conclusion  by  a  most 
ingenious  and  beautiful  experiment ;  and  although  his  conclusions  do 

not  rest  on  absolutely  certain 
ground,  they  at  least  open  the 
way  to  a  decisive  test.  The 
Hertwig  brothers  showed  that 
the  eggs  of  sea-urchins  might 
be  enucleated  by  shaking,  and 
that  spermatozoa  would  enter 
the  enucleated  fragments  and 
cause  them  to  segment.  Boveri 
proved  that  such  fragments 
would  even  give  rise  to  dwarf 
larvae,  indistinguishable  from 
the  normal  in  general  appear- 
ance and  differing  from  the 
latter  only  in  size  and  in  the 
very  significant  fact  that  their 
nuclei  contain  only  half  the  nor- 
mal number  of  chromosomes. 
Now,  by  fertilizing  enucleated 
egg-fragments  of  one  species 
(Sphcsrechinns  gramilaris)  with 
the  spermatozoa  of  another 
{Echinus  inicrotubercidattis),  Bo- 
veri obtained  in  a  few  instances 
dwarf  Plutei  sJiowing  purely 
paternal  characteristics  (I'ig. 
116).  From  this  he  concluded 
that  the  maternal  cytoplasm  has 
no  determining  effect  on  the 
offspring,  but  supplies  only  the 
material  in  which  the  sperm- 
nucleus  operates.  Inheritance 
is,  therefore,  effected  by  the 
nucleus  alone. ^  Boveri's  result 
is  unfortunately  not  quite  conclusive,  as  has  been  pointed  out 
by  Seeliger  and  Morgan,  yet  his  extensive  experiments  establish,  I 
think,  a  strong  presumption  in  its  favour.  Should  they  be  positively 
confirmed,  they  would  furnish  a  practical  demonstration  of  inheritance 
through  the  nucleus. 


Fig.  116.  —  Normal  and  dwarf  larvae  of  the 
sea-urchin.     [Boveri.] 

A.  Dwarf  Pluteus  arising  from  an  enucleated 
*-'ga-fragment  ot  Sphesrechinus granulans,  fertilized 
with  spermatozoon  of  Echinus  niicrotvberculatus, 
2ind  s\\o\Nmg  p7irely paternal  characters.  B.  Nor- 
mal Pluteus  of  Echinus  rntcrotuberculatus. 


1  The  centrosome  is  left  out  of  account,  since  it  is  rr< 


(lue 


itlv  derived  from  «me  sex  oulv. 


THE    CENTROSOME    ,  259 


5.    77ie  Nucleus  in  Maturation 

Scarcely  less  convincing,  finally,  is  the  contrast  between  nucleus 
and  cytoplasm  in  the  maturation  of  the  germ-cells.  It  is  scarcely 
an  exaggeration  to  say  that  the  whole  process  of  maturation,  in  its 
broadest  sense,  renders  the  cytoplasm  of  \he  germ-cells  as  unlike, 
the  nuclei  as  like,  as  possible.  The  latter  undergo  a  series  of  com- 
plicated changes  which  are  expressly  designed  to  establish  a  perfect 
equivalence  between  them  at  the  time  of  their  union,  and,  more  re- 
motely, a  perfect  equality  of  distribution  to  the  embryonic  cells. 
The  cytoplasm,  on  the  other  hand,  undergoes  a  special  and  per- 
sistent differentiation  in  each  to  effect  a  secondary  division  of  labour 
between  the  germ-cells.  When  this  is  correlated  with  the  fact  that 
the  germ-cells,  on  the  whole,  have  an  equal  effect  on  the  specific 
character  of  the  embryo,  we  are  again  forced  to  the  conclusion  that 
this  effect  must  primarily  be  sought  in  the  nucleus,  and  that  the 
cytoplasm  is  in  a  sense  only  its  agent. 


C.     The  Centrosome 

Nearly  all  investigators  have  now  accepted  Van  Beneden's  and 
Boveri's  conclusion  that  the  ce7ttrosome  is  an  organ  for  cell-division^ 
and  that  in  this  sense  it  represents  the  dynamic  centre  of  the  cell  (cf. 
p.  56).  This  is  most  clearly  shown  in  the  ordinary  fertilization  of  the 
ovum,  in  which  process,  as  Boveri  has  insisted,  it  is  the  centrosome 
that  is  the  fertilizing  element  par  excellence^  since  its  introduction 
into  the  ^gg  confers  upon  the  latter  the  power  of  division,  and  hence 
of  develojDment.  Boveri's  interesting  observations  on  *'  partial  fertil- 
ization" in  the  sea-urchin  referred  to  at  p.  140  afford  a  beautiful  illus- 
tration of  this  point.  In  certain  exceptional  cases  the  ^gg  may  divide 
before  conjugation  of  the  germ-nuclei  has  occurred,  the  sperm-nucleus 
lying  passive  in  the  cytoplasm  until  after  the  first  cleavage  and  then 
conjugating  with  one  of  the  nuclei  of  the  two-celled  stage.  The  ^gg 
is  \i^Y^  fertilized — i.e.  rendered  capable  of  division  —  by  the  centro- 
some, which  separates  from  the  sperm-nucleus,  approaches  the  egg- 
nucleus,  and  gives  rise  to  the  cleavage-amphiaster  as  usual. 

Again,  Boveri  has  observed  that  the  segmenting  ovum  of  Ascaris 
sometimes  contains  a  supernumerary  centrosome  that  does  not  enter 
into  connection  with  the  chromosomes,  but  lies  alone  in  the  cytoplasm 
(Fig.  117).  Such  a  centrosome  forms  an  independent  centre  of  divi- 
sion, the  cell  dividing  into  three  parts,  two  of  which  are  normal 
blastomeres,  while  the  third  contains  only  the  centrosome  and  attrac- 


26o      SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

tion-sphere.  The  fate  of  such  eggs  was  not  determined,  but  they 
form  a  complete  demonstration  that  it  is  the  centrosome  and  not  the 
nucleus  that  is  the  active  centre  of  cell-division  in  the  cell-body. 
Scarcely  less  conclusive  is  the  case  of  dispcrmic  eggs  in  sea-urchins. 
In  such  eggs  both  sperm-nuclei  conjugate  with  the  egg-nucleus,  and 
both  sperm-centrosomes  divide  (Fig.  ii8).  The  cleavage-nucleus, 
therefore,  arises  by  the  union  of  three  nuclei  and  fojir  centrosomes. 
Such  eggs  invariably  divide  at  the  first  cleavage  into  four  equal  blas- 
tomeres,  each  of  which  receives  one  of  the  centrosomes.  The  latter 
must,  therefore,  be  the  centres  of  division. ^ 

The  statement  that  the  centrosome  is  an  organ  for  cell-division 
does  not,  however,  express  the  whole  truth ;  for  in  leucocytes  and 
pigment-cells  the  astral  system  formed  about  it  is  devoted,  as  there  is 
good  reason  to  believe,  not  to  cell-division,  but  to  movements  of  the 


Fig.  117.  —  Eggs  of  Ascaris  with  supernumerary  centrosome.     [BOVERI.] 
A.  First  cleavage-spindle  above,  isolated  centrosome  below.    B.  Result  of  the  ensuing  division. 


cell-body  as  a  whole ;  and,  moreover,  amitotic  division  may  appar- 
ently take  place  independently  of  the  centrosome.  The  role  of  the 
centrosome  and  attraction-sphere  in  gland-cells  (where  they  are  some- 
times very  large)  and  in  the  nerve-cells  is  still  wholly  problematical. 
It  would  seem,  therefore,  that  the  primary  function  of  the  centrosome 
is  to  organize  an  astral  system,  of  which  it  forms  the  focus,  that  is 
primarily  an  apparatus  for  mitotic  division,  but  may  secondarily 
become  devoted  to  other  functions.  The  nature  of  the  energy  by 
which  this  organization  takes  place  is  almost  wholly  in  the  dark. 
The  extraordinary  resemblance  of  the  amphiaster  to  the  lines  of 
force  in  a  magnetic  field  has  impressed  many  observers,  but  Roux 
has  proved  that  the  axis  of  the  mitotic  figure  is  not  affected,  during 
its  formation,  by  a  powerful  electro-magnet.     The  molecules  or  micro- 


1  This  phenomenon  was  first  observed  by  Ilertwig,  and  afterwards  by  Driesch. 
repeatedly  observed  the  internal  changes  in  the  living  eggs  of  Toxopneustes. 


1  have 


SUMMARY  AND    CONCLUSION 


261 


somes  of  the  fibres  must  be  in  some  manner  polarized  by  an  influence 
emanating  from  the  centrosome,  but  in  the  present  state  of  know- 
ledge it  would  be  useless  to  speculate  on  the  nature  of  this  influence. 
One  fact,  however,  should  be  borne  in  mind,  namely,  that  the  centro- 
some differs  chemically  from  the  substance  of  the  fibres  as  shown  by 
its  staining-reactions ;  and  this  may  form  a  clue  to  the  further  inves- 
tigation of  this  most  interesting  problem. 

The  principal  point  in  connection  with  our  present  theme  is  that 
the  centrosome  cannot  be  regarded  as  taking  any  important  part  in 


B 


C 


Fig.  118.  —  Cleavage  of  dispermic  ^^<g  of  Toxopneustcs. 
A.  One  sperm-nucleus  has  united  with  the  egg-nucleus,  shown  2A  a,  b;  the  other  lies  above. 
Both  sperm-asters  have  divided  to  form  amphiasters  {a,  b  and  c,  d).     B.  The  cleavage-nucleus 
formed  by  union  of  the  three  germ-nuclei,  is  surrounded  by  the  four  asters.     C.  Result  of  the  first 
cleavage,  the  four  blastomeres  lettered  to  correspond  with  the  four  asters. 

the  general  metabolism  of  the  cell,  nor  can  it  be  an  organ  of  inheri- 
tance ;  for  on  the  one  hand  it  is  absent  or  so  small  as  to  be  indistin- 
guishable in  many  actively  metabolizing  cells,  such  as  those  of  the 
pancreas  or  kidney,  or  the  older  ovarian  eggs,  and,  on  the  other  hand, 
in  fertilization  it  may  be  derived  from  one  sex  only.  The  conclusion 
regarding  inheritance  would  not  be  invalidated,  even  if  it  could  be 
positively  shown  that'  in  some  cases  both  germ-cells  might  contribute 
a  centrosome ;  for  a  single  case  of  its  one-sided  origin  would  be  con- 
clusive, and  many  such  are  actually  known. 


D.     Summary  and  Conclusion 


All  of  the  facts  reviewed  in  the  foregoing  pages  converge,  I  think, 
to  the  conclusion  drawn  by  Claude  Bernard,  that  the  nucleus  is  the 
formative  centre  of  the  cell  in  a  chemical  sense,  and  through  this  is 
the  especial  seat  of  the  formative  energy  in  a  morphological  sense. 
That  the  nucleus  has  such  a  significance  in  synthetic  metabolism  is 
proved  by  the  fact  that  digestion  and  absorption  of  food,  growth,  and 


262       SOME  ASPECTS   OF  CELL-CHEMISTRY  AND    CELL-PHYSIOLOGY 

secretion  cease  with  its  removal  from  the  cytoplasm,  while  destructive 
metabolism  may  long  continue  as  manifested  by  the  phenomena  of 
irritability  and  contractility.  It  is  indicated  by  the  position  and  move- 
ments of  the  nucleus  in  relation  to  the  food-supply  and  to  the  forma- 
tion of  specific  cytoplasmic  products.  It  harmonizes  with  the  fact, 
now  universally  admitted,  that  active  exchanges  of  material  go  on 
between  nucleus  and  cytoplasm.  The  periodic  changes  of  staining- 
capacity  undergone  by  the  chromatin  during  the  cycle  of  cell-life, 
taken  in  connection  with  the  researches  of  physiological  chemists  on 
the  chemical  composition  and  staining-reactions  of  the  nuclein-series, 
indicate  that  the  substance  known  as  niLclcic  acid  plays  a  leading  part 
in  the  constructive  process.  During  the  vegetative  phase  of  the  cell 
this  substance  appears  to  enter  into  combination  with  proteid  or 
albuminous  substance  to  form  a  nuclein.  During  its  mitotic  or  repro- 
ductive phase  the  albumin  is  split  off,  leaving  the  substance  of  the 
chromosomes  as  nearly  pure  nucleic  acid.  When  this  is  correlated 
with  the  fact  that  the  sperm-nucleus,  which  brings  with  it  the  pater- 
nal heritage,  likewise  consists  of  nearly  pure  nucleic  acid,  the  pos- 
sibility is  opened  that  this  substance  may  be  in  a  chemical  sense  not 
only  the  formative  centre  of  the  nucleus  but  also  a  primary  factor  in 
the  constructive  processes  of  the  cytoplasm. 

The  role  of  the  nucleus  in  constructive  metabolism  is  intimately 
related  with  its  role  in  morphological  synthesis  and  thus  in  inheri- 
tance ;  for  the  recurrence  of  similar  morphological  characters  must  in 
the  last  analysis  be  due  to  the  recurrence  of  corresponding  forms  of 
metabolic  action  of  which  they  are  the  outward  expression.  That 
the  nucleus  is  in  fact  a  primary  factor  in  morphological  as  well  as 
chemical  synthesis  is  demonstrated  by  experiments  on  unicellular 
plants  and  animals,  which  prove  that  the  power  of  regenerating  lost 
parts  disappears  with  its  removal,  though  the  enucleated  fragment 
may  continue  to  live  and  move  for  a  considerable  period. 

This  fact  establishes  the  presumption  that  the  nucleus  is,  if  not  the 
actual  seat  of  the  formative  energy,  at  least  the  controlling  factor  in 
that  energy,  and  hence  the  controlling  factor  in  inheritance.  This 
presumption  becomes  a  practical  certainty  when  we  turn  to  the 
facts  of  maturation,  fertilization,  and  cell-division.  All  of  these  con- 
verge to  the  conclusion  that  the  chromatin  is  the  most  essential  ele- 
ment in  development.  In  maturation  the  germ-nuclei  are  by  an 
elaborate  process  prepared  for  the  subsequent  union  of  equivalent 
chromatic  elements  from  the  two  sexes.  By  fertilization  these  ele- 
ments are  brought  together  and  by  mitotic  division  distributed  with 
exact  equality  to  the  embryonic  cells.  The  result  proves  that  the 
spermatozoon  is  as  potent  in  inheritance  as  the  ovum,  though  the 
latter   contributes  an  amount   of    cytoplasm  which  is  but   an   infini- 


SUMMARY  AND    CONCLUSION  263 

tesinial  fraction  of  that  supplied  by  the  ovum.  The  centrosome, 
finally,  is  excluded  from  the  process  of  inheritance,  since  it  may  be 
derived  from  one  sex  only. 


LITERATURE.     VII 

Bernard,  Claude.  —  Lepons  sur  les  Phenomenes  de  la  Vie:    ist  ed.   1878;    2d  ed. 

1885.     Paris. 
Chittenden,  R.  H.  —  Some  Recent  Chemico-physiological  Discoveries  regarding  the 

Cell :  Am.  Nat.,  XXVIII.,  Feb.,  1894. 
Haberlandt,  G. — Uber  die  Beziehungen  zwischen  Funktion  und  Lage  des  Zellkerns. 

Fischer,  1887. 
Halliburton,  W.  D.  —  A  Text-book  of  Chemical  Physiology  and  Pathology.     London., 

1891. 
Id.  —  The  Chemical  Physiology  of  the  Cell  {Gouldstonian  Lectures^:  Brit.  Med. 

Journ .     1 893 . 
Hammarsten,  0.  —  Lehrbuch  der  physiologische  Chemie.     3d  ed.     Wiesbaden,  1895. 
Hertwig,  0.  &  R.  —  Uber  den  Befruchtungs-  und  Teilungsvorgang  des  tierischen 

Eies  unter  dem  Einfluss  ausserer  Agentien.     Jena,  1887. 
Kolliker,  A.  —  Das  Karyoplasma  und  die  Vererbung,  eine  Kritik  der  Weismann'schen 

Theorie  von  der  Kontinuitat  des  Keimplasmas :  Zeitschr.  wiss.  Zo'dl.,  XLIV. 

1886. 
Korschelt,  E.  —  Beitrage  sur  Morphologie  und   Physiologie  des  Zell-kernes :  Zo'dl. 

Jahrb.  Anat.  u.  Ontog.,  IV.     1889. 
Kossel,  A.  —  (jber  die  chemische  Zusammensetzung  der  Zelle  :  Arch.  Anat.  u.  Phys. 

1891. 
Lilienfeld,  L.  —  Uber   die   Wahlverwandtschaft   der   Zellelemente   zu   Farbstoffen : 

Arch.  Anat.  u.  Phys.     1893. 
Maliatti,  H.  —  Beitrage  zur  Kenntniss  der  Nucleine :  Zeitschr.  Phys.  Chem.,  XVI. 

1891. 
Riickert,  J.  —  Zur  Entwicklungsgeschichte  des  Ovarialeies  bei  Selachiern :  An.  Anz.., 

VII.     1892. 
Sachs,  J.  —  Vorlesungen  liber  Pflanzen-physiologie.     Leipzig,  1882. 
Id.  —  Stoff  und  Form  der  Pflanzen-organe  :   Gesaininelte  Abhandbmgen,  II.     1893. 
Verworn,  M.  —  Die  Physiologische  Bedeutung  des  Zellkerns:  Arch,  fiir  die  Ges. 

Phys.,XL\.     1892. 
Id. —  Allgemeine  Physiologie.     Jena,  1895. 

Zacharias,  E.  —  Uber  Chromatophilie  :  Ber.  d.  deutsch.  Bot.  Ges.     1893. 
Id.  —  Uber  des  Verhalten  des  Zellkerns  in  wachsenden  Zellen  :  Flora,  81.     1895. 
Whitman,  C.  0.  —  The  Seat  of  Formative  and  Regenerative  Energy  :  Journ.  Morph., 

II.     1888. 


CHAPTER   VIII 

CELL-DIVISION   AND   DEVELOPMENT 

"Wir  konnen  demnach  endlich  den  Satz  aufstellen,  dass  sammtliche  im  entwickelten 
Zustande  vorhandenen  Zellen  oder  Aequivalente  von  Zellen  durch  eine  fortschreitende 
Gliederung  der  Eizelle  in  morphologisch  ahnliche  Elemente  entstehen,  und  dass  die  in  einer 
embryonischen  Organ-Anlage  enthaltenden  Zellen,  so  gering  auch  ihre  Zahl  sein  mag, 
dennoch  die  ausschliessliche  ungegliederte  Anlage  fUr  sammtliche  Formbestandtheile  der 
spateren  Organe  enthalten."  Remak.i 

Since  the  early  work  of  Kolliker  and  Remak  it  has  been  recog- 
nized that  the  cleavage  or  segmentation  of  the  ovum,  with  which 
the  development  of  all  higher  animals  begins,  is  nothing  other  than 
a  rapid  series  of  mitotic  cell-divisions  by  which  the  egg  splits  up 
into  the  elements  of  the  tissues.  Th'is  process  is  merely  a  contin- 
uation of  that  by  which  the  germ-cell  arose  in  the  parental  body. 
A  long  pause,  however,  intervenes  during  the  latter  period  of  its 
ovarian  life,  during  which  no  divisions  take  place.  Throughout  this 
period  the  egg  leads,  on  the  whole,  a  somewhat  passive  existence, 
devoting  itself  especially  to  the  storage  of  potential  energy  to  be  used 
during  the  intense  activity  that  is  to  come.  Its  power  of  division 
remains  dormant  until  the  period  of  full  maturity  approaches.  The 
entrance  of  the  spermatozoon,  bringing  with  it  a  new  centrosome, 
arouses  in  the  egg  a  new  phase  of  activity.  Its  power  of  division, 
which  may  have  lain  dormant  for  months  or  years,  is  suddenly  raised 
to  the  highest  pitch  of  intensity,  and  in  a  very  short  time  it  gives 
rise  by  division  to  a  myriad  of  descendants  which  are  ultimately 
differentiated  into  the  elements  of  the  tissues. 

The  divisions  of  the  egg  during  cleavage  are  exactly  comparable 
with  those  of  tissue-cells,  and  all  of  the  essential  phenomena  of 
mitosis  are  of  the  same  general  character  in  both.  But  for  two 
reasons  the  cleavage  of  the  egg  possesses  a  higher  interest  than 
any  other  case  of  cell-division.  First,  the  egg-cell  gives  rise  by  divi- 
sion not  only  to  cells  like  itself,  as  is  the  case  with  most  tissue-cells, 
but  also  to  many  other  kinds  of  cells.  The  operation  of  cleavage  is 
therefore  immediately  connected  with  the  process  of  differentiation, 

1  Untersuchungen,  1855,  p.  140. 
264 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  265 

which  is  the  most  fundamental  phenomenon  in  development.  Second, 
definite  relations  may  often  be  traced  between  the  planes  of  division 
and  the  structural  axes  of  the  adult  body,  and  these  relations  are 
sometimes^so  clearly  marked  and  appear  so  early  that  with  the  very 
first  cleavage  the  position  in  which  the  embryo  will  finally  appear  in 
the  Q,gg  may  be  exactly  predicted.  Such  "  promorphological  "  rela- 
tions of  the  segmenting  ^^'g  possess  a  very  high  interest  in  their 
bearing  on  the  theory  of  germinal  localization  and  on  account  of  the 
light  which  they  throw^  on  the  conditions  of  the  formative  process. 

The  present  chapter  is  in  the  main  a  prelude  to  that  which 
follows,  its  purpose  being  to  sketch  some  of  the  external  features 
of  early  development  regarded  as  particular  expressions  of  the  gen- 
eral laws  of  cell-division.  For  this  purpose  we  may  consider  the 
cleavage  of  the  ovum  under  two  heads,  namely :  — 

1.  TJie  Geometrical  Relations  of  Cleavage-forms ^  with  reference 
to  the  general  laws  of  cell-division. 

2.  T/ie  Promorphological  Relations  of  the  blastomeres  and  cleav- 
age-planes to  the  parts  of  the  adult  body  to  which  they  give  rise. 


A.     Geometrical  Relations  of  Cleavage-forms 

The  geometrical  relations  of  the  cleavage-planes  and  the  relative 
size  and  position  of  the  cells  vary  endlessly  in  detail,  being  modified 
by  innumerable  mechanical  and  other  conditions,  such  as  the  amount 
and  distribution  of  the  inert  yolk  or  deutoplasm,  the  shape  of  the 
ovum  as  a  whole,  and  the  like.  Yet  all  the  forms  of  cleavage  are 
variants  of  a  single  type  which  has  been  moulded  this  way  or  that 
by  special  conditions,  and  which  is  itself  an  expression  of  two  general 
laws  of  cell-division,  first  formulated  by  Sachs  in  the  case  of  plant- 
cells.     These  are : 

1.  The  cell  typically  tends  to  divide  into  eqnal parts. 

2.  Each  7tezv  plane  of  division  tends  to  intersect  the  preceding  plane 
at  a  right  angle. 

In  the  simplest  and  least  modified  forms  the  direction  of  the 
cleavage-planes,  and  hence  the  general  configuration  of  the  cell- 
system,  depends  on  the  general  form  of  the  dividing  mass;  for,  as 
Sachs  has  shown,  the  cleavage-planes  tend  to  be  either  vertical  to  the 
surface  {anticlines^  or  parallel  to  it  {periclijies).  Ideal  schemes  of 
division  may  thus  be  constructed  for  various  geometrical  figures.  In 
a  flat  circular  disc,  for  example,  the  anticlinal  planes  pass  through 
the  radii;  the  periclines  are  circles  concentric  with  the  periphery.     If 


266 


CELL-Dl  I  'ISI  ON  AND  DE  VEL  0PM ENT 


the  disc  be  elongated  to  form  an  ellipse,  the  periclines  also  become 
ellipses,  while  the  anticlines  are  converted  into  hyperbolas  confocal 
with  the  periclines.  If  it  have  the  form  of  a  parabola,  the  periclines 
and  anticlines  form  two  systems  of  confocal  parabolas  intersecting  at 


Fig.  119. —  Geometrical  relations  of  cleavage-planes  in  growing  plant-tissues.  [From  Sachs, 
after  various  authors.] 

A.  Flat  ellipsoidal  germ-disc  of  Melobesia  (Rosanoff)  ;  nearly  typical  relation  of  elliptic 
periclines  and  hyperbolic  anticlines.  B.  C.  Apical  view  of  terminal  knob  on  epidermal  hair  of 
Pinguicola.  B.  shows  the  ellipsoid  type,  C.  the  circular  (spherical  type),  somewhat  modified 
(only  anticlines  present),  D.  Growing  point  of  Saivinia  (Pringsheim)  ;  typical  ellipsoid  type, 
the  single  pericline  is  however  incomplete.  E.  Growing  point  of  Azolla  (Strasburger) ;  circular 
or  spheroidal  type  transitional  to  ellipsoidal.  F.  Root-cap  of  Equisetum  (Nageli  and  Leitgeb)  ; 
modified  circular  type.  G.  Cross-section  of  leaf-vein,  Trichomanes  (Prantl)  ;  ellipsoidal  type  with 
incomplete  periclines.  H.  Embryo  of  Alisma ;  typical  ellipsoid  type,  pericline  incomplete  only 
at  lower  side.  /.  Growing  point  of  bud  of  the  pine  (Adies)  ;  typical  paraboloid  type,  both  anti- 
clines and  periclines  having  the  form  of  parabolas  (Sachs). 


right  angles.     All  these  schemes  are,  mutatis  mutandis,  directly  con- 
vertible into  the  corresponding  solid  forms  in  three  dimensions. 

Sachs  has  shown  in  the  most  beautiful  manner  that  all  the  above 
ideal  types  arc  closely  approximated  in  nature,  and  Rauber  has  applied 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  26/ 

the  same  principle  to  the  cleavage  of  animal  cells.  The  discoid  or 
spheroid  form  is  more  or  less  nearly  realized  in  the  thalloid  growths 
of  various  lower  plants,  in  the  embryos  of  flowering  plants,  and 
elsewhere  (Fig.  119).  The  paraboloid  form  is  according  to  Sachs 
characteristic  of  the  growing  points  of  many  higher  plants;  and 
here  too  the  actual  form  is  remarkably  similar  to  the  ideal  scheme 
(Fig.  119,  /). 

For  our  purpose  the  most  important  form  is  the  sphere,  which  is 
the  typical  shape  of  the  egg-cell;  and  all  forms  of  cleavage  are  deriv- 
atives of  the  typical  division  of  a  sphere  in  accordance  with  Sachs's 
laws.  The  ideal  form  of  cleavage  would  here  be  a  succession  of 
rectangular  cleavages  in  the  three  dimensions  of  space,  the  anticlines 
passing  through  the  centre  so  as  to  split  the  ^gg  in  the  initial  stages 
successively  into  halves,  quadrants,  and  octants,  the  periclines  being 
parallel  to  the  surface  so  as  to  separate  the  inner  ends  of  these  cells 
from  the  outer.  No  case  is  known  in  which  this  order  is  accurately 
followed  throughout,  and  the  periclinal  cleavages  are  of  compara- 
tively rare  occurrence,  being  found  as  a  regular  feature  of  the  early 
cleavage  only  in  those  cases  where  the  primary  germ-layers  are  sepa- 
rated by  delamination.  The  simplest  and  most  typical  forfn  of  egg- 
cleavage  occurs  in  eggs  like  those  of  echinoderms,  which  are  of 
spherical  form,  and  in  which  the  deutoplasm  is  small  in  amount  and 
equally  distributed  through  its  substance.  Such  a  cleavage  is  beauti- 
fully displayed  in  the  ^gg  of  the  holothurian  Synapta,  as  shown  in 
the  diagrams.  Fig.  120,  constructed  from  Selenka's  drawings.^  The 
first  cleavage  is  vertical,  or  meridional^  passing  through  the  egg-axis 
and  dividing  the  ^gg  into  equal  halves.  The  second,  which  is  also 
meridional,  cuts  the  first  plane  at  right  angles  and  divides  the  ^gg 
into  quadrants.  The  third  is  horizontal,  or  equatorial,  dividing  the 
Qgg  into  equal  octants.  The  order  of  division  is  thus  far  exactly 
that  demanded  by  Sachs's  law  and  agrees  precisely  with  the  cleavage 
of  various  kinds  of  spherical  plant-cells.  The  later  cleavages  depart 
from  the  ideal  type  in  the  absence  of  periclinal  divisions,  the  embryo 
becoming  hollow,  and  its  wall  consisting  of  a  single  layer  of  cells  in 
which  anticlinal  cleavages  occur  in  regular  rectangular  succession. 
The  fourth  cleavage  is  again  meridional,  giving  two  tiers  of  eight 
cells  each;  the  fifth  is  horizontal,  dividing  each  tier  into  an  upper 
and  a  lower  layer.  The  regular  alternation  is  continued  up  to  the 
ninth  division  (giving  512  cells),  when  the  divisions  pause  while  the 
gastrulation  begins.     In  later  stages  the  regularity  is  lost. 

This  simple  and  regular  mode  of  division  forms  a  type  to  which 
nearly  all  forms  of  cleavage  may  be  referred  ;  but  the  order  and  form 

1  Cf.  also  Fig.  3. 


268 


CELL-DIVISION  AND  DEVELOPMENT 


of  the  divisions  is  endlessly  varied  by  special  conditions.     These 
modifications  are  all  referable  to  the  three  following  causes :  — 

1.  Disturbances  in  the  rhythm  of  division. 

2.  Displacement  of  the  cells. 

3.  Unequal  division  of  the  cells. 

The  first  of  these  requires  little  comment.  Nothing  is  more  com- 
mon than  a  departure  from  the  mathematical  regularity  of  division. 
The  variations  are  sometimes  quite  irregular,  sometimes  follow  a 
definite  law,  as,  for  instance,  in  the  annelid  Nereis  (Fig.  122),  where 
the  typical  succession  in  the  number  of  cells  is  with  great  constancy 


Fig.  120.  —  Cleavage  of  the  ovum  in  the  holothurian  Synapta  (slightly  schematized).  [After 
Selenka.] 

A-E.  Successive  cleavages  to  the  32-cell  stage.    F.  Blastula  of  128  cells. 

2,  4,  8,  16,  20,  23,  29,  32,  37,  38,  41,  42,  after  which  the  order  is  more 
or  less  variable.  The  meaning  of  such  variations  in  particular  cases 
is  not  very  clear.  They  are  certainly  due  in  part  to  variations  in  the 
amount  of  deutoplasm ;  for,  as  Balfour  long  since  pointed  out  ('75), 
the  rapidity  of  division  in  any  part  of  the  ovum  is  in  general  inversely 
proportional  to  the  amount  of  deutoplasm  it  contains.  Exceptions 
to  this  law  are,  however,  known. 

The  second  series  of  modifications,  due  to  displacements  of  the 
cells,  are  probably  due  to  mutual  pressure,  however  caused,^  which 

^  The  pressure  is  probably  due  primarily  to  an  attraction  between  the  cells  {cytotropic in 
of  Roux),  but  may  be  increased  by  the  presence  of  membranes,  by  turgor,  or  by  special 
processes  of  growth. 


GEOMETRICAL  RELATIONS   OF  CLEAVAGE-FORMS 


269 


leads  them  to  take  up  the  position  of  least  resistance  or  greatest 
economy  of  space.  In  this  regard  the  behaviour  of  tissue-cells  in 
general  has  been  shown  to  conform  on  the  whole  to  that  of  elastic 
spheres,  su6h  as  soap-bubbles  when  massed  together  and  free  to 
move.  Such  bodies,  as  Plateau  and  Lamarle  have  shown,  assume  a 
polyhedral  form  and  tend  towards  such  an  arrangement  that  the  area 


Fig.  121.  —  Cleavage  oi  Polygordius,  from  life. 
A.  Four-cell  stage,  from  above.    B.  Corresponding  view  of  8-cell  stage, 
same  (contrast  Fig.  120,  C).    D.  Sixteen-cell  stage  from  the  side. 


C.  Side  view  of  the 


of  surface-contact  betzvecn  t/iein  is  a  minimttm.  Spheres  in  a  mass 
thus  tend  to  assume  the  form  of  interlocking  polyhedrons  so  arranged 
that  three  planes  intersect  in  a  line,  while  four  lines  and  six  planes 
meet  at  a  point.  If  arranged  in  a  single  layer  on  an  extended  sur- 
face they  assume  the  form  of  hexagonal  prisms,  three  planes  meeting 
along  a  line  as  before.  Both  these  forms  are  commonly  shown  in  the 
arrangement  of  the  cells  of  plant  and  animal  tissues ;  and  Berthold 


2/0  CELL-DIVISION  AND  DEVELOPMENT 

('86)  and  Errara  ('86,  '^y)  have  pointed  out  that  in  almost  all  cases 
the  cells  tend  to  alternate  or  interlock  so  as  to  reduce  the  contact-area 
to  a  minimum.  Thus  arise  many  of  the  most  frequent  modifications 
of  cleavage.  Sometimes,  as  in  Synapta,  the  alternation  of  the  cells  is 
effected  through  displacement  of  the  blastomeres  after  their  forma- 
tion. More  commonly  it  arises  during  the  division  of  the  cells  and 
may  even  be  predetermined  by  the  position  of  the  mitotic  figures 
before  the  slightest  external  sign  of  division.  Thus  arises  that  form 
of  cleavage  known  as  the  spiral,  oblique,  or  alternating  type,  where 
the  blastomeres  interlock  during  their  formation  and  lie  in  the  posi- 
tion of  least  resistance  from  the  beginning.  This  form  of  cleavage, 
especially  characteristic  of  many  worms  and  mollusks,  is  typically 
shown  by  the  ^%^  of  Polygordiiis  (Fig.  I2i).  The  four-celled  stage  is 
nearly  like  that  of  Synapta,  though  even  here  the  cells  slightly  inter- 
lock. The  third  division  is,  however,  oblique,  the  four  upper  cells 
being  virtually  rotated  to  the  right  (with  the  hands  of  a  watch)  so  as 
to  alternate  with  the  four  lower  ones.  The  fourth  cleavage  is  like- 
wise oblique,  but  at  right  angles  to  the  third,  so  that  all  of  the  cells 
interlock  as  shown  in  Fig.  I2i,  D.  This  alternation  regularly  recurs 
in  the  later  cleavages. 

This  form  of  cleavage  beautifully  illustrates  Sachs's  second  law 
operating  under  modified  conditions,  and  the  conclusion  is  irresistible 
that  the  modification  is  at  bottom  a  result  of  the  same  forces  as  those 
operating  in  the  case  of  soap-bubbles.  In  many  worms  and  mollusks 
the  obliquity  of  cleavage  appears  still  earlier,  at  the  second  cleavage, 
the  four  cells  being  so  arranged  that  two  of  them  meet  along  a  "cross- 
furrow"  at  the  lower  pole  of  the  ^^g,  while  the  other  two  meet  at  the 
upper  pole  along  a  similar,  though  often  shorter,  cross-furrow  at  right 
angles  to  the  lower  {e.g.  in  Nereis,  Fig.  122).  It  is  a  curious  fact 
that  the  direction  of  the  displacement  is  extremely  constant,  the 
upper  quartet  in  the  eight-cell  stage  being  rotated  in  all  but  a  few 
cases  to  the  right,  or  with  the  hands  of  a  watch.  Crampton  ('94)  has 
discovered  the  remarkable  fact  that  in  Physa^  a  gasteropod  having  a 
reversed  or  sinistral  shell,  the  whole  order  of  displacement  is  likewise 
reversed. 

The  third  class  of  modifications,  due  to  unequal  division  of  the  cells, 
leads  to  the  most  extreme  types  of  cleavage.  Such  divisions  appear 
sooner  or  later  in  all  forms  of  cleavage,  the  perfect  equality  so  long 
maintained  in  Sytiapta  being  a  rare  phenomenon.  The  period  at 
which  the  inequality  first  appears  varies  greatly  in  different  forms. 
In  Polygordiiis  (Fig.  121)  the  first  marked  inequality  appears  at  the 
fifth  cleavage;  in  sea-urchins  it  appears  at  the  fourth  (Fig.  3);  in 
Amphioxus  at  the  third  (Fig.  123);  in  the  tunicate  Clavelina  at  the 
second  (Fig.  126);  in  Nereis  at  the  first  division  (Figs.  43,  122).     The 


GEOMETRICAL  RELATIONS   OF  CLEAVAGE-FORMS 


271 


extent  of  the  inequality  varies  in  like  manner.  Taking  the  third 
cleavage  as  a  type,  we  may  trace  every  transition  from  an  equal  divi- 
sion (echinoderms,  Polygordius),  through  forms  in  which  it  is  but 
slightly  marked  {Amphioxus,  frog),  those  in  which  it  is  conspicuous 
{Nereis,  L^mttcBa,  Polyclades,  Petromyzon,  etc.),  to  forms  such  as  Clep- 
sine,  where  the  cells  of  the  upper  quartet  are  so  minute  as  to  appear 
like  mere  buds  from  the  four  large  lower  cells  (Fig.  123).     At  the 


Fig.  122.  —  Cleavage  of  Nereis.  An  example  of  a  spiral  cleavage,  unequal  from  the  beginning 
and  of  a  marked  mosaic-like  character. 

A.  Two-cell  stage  (the  circles  are  oil-drops).  B.  Four-cell  stage;  the  second  cleavage-plane 
passes  through  the  future  median  plane,  C.  The  same  from  the  right  side.  D.  Eight-cell  stage. 
E.  Sixteen  cells  ;  from  the  cells  marked  /  arises  the  prototroch  or  larval  ciliated  belt,  from  X  the 
ventral  nerve-cord  and  other  structures,  from  D  the  mesoblast-bands,  the  germ-cells,  and  a  part  of 
the  alimentary  canal.  F.  Twenty-nine-cell  stage,  from  the  right  side;  p.  girdle  of  prototrochal 
cells  which  give  rise  to  the  ciliated  belt. 


extreme  of  the  series  we  reach  the  partial  or  meroblastic  cleavage, 
such  as  occurs  in  the  cephalopods,  in  many  fishes,  and  in  birds  and 
reptiles.  Here  the  lower  hemisphere  of  the  Qgg  does  not  divide  at 
all,  or  only  at  a  late  period,  segmentation  being  confined  to  a  disc- 
like region  or  blastoderm  at  one  pole  of  the  Qgg  (Fig.  124). 

Very  interesting  is  the  case  of  the  tcloblasts  or  pole-cells  character- 
istic of  the  development  of  many  annelids  and  mollusks  and  found  in 
some  arthropods.  These  remarkable  cells  are  large  blastomeres,  set 
aside  early  in  the  development,  which  bud  forth  smaller  cells  in  reg- 


72 


CELL-DIVISION  AND   DEVELOPMENT 


ular  succession  at  a  fixed  point,  thus  giving  rise  to  long  cords  of  cells 
(Fig.  125).  The  teloblasts  are  especially  characteristic  of  apical 
growth,  such  as  occurs  in  the  elongation  of  the  body  in  annelids,  and 
they  are  closely  analogous  to  the  apical  cells  situated  at  the  growing 
point  in  many  plants,  such  as  the  ferns  and  stoneworts. 


Fig.  123.  —  The  8-cell  stage  of  four  different  animals  showing  gradations  in  the  inequality  of 
the  third  cleavage. 

A.  The  leech  Clepsin.e  (Whitman).  B.  The  chaetopod  Rhynchelmis  (Vejdovsky).  C.  The 
lamellibranch  Unio  (Lillie).     D.  Amphioxus. 


Unequal  division  still  awaits  an  explanation.  The  fact  has  already 
been  pointed  out  (p.  51)  that  the  inequality  of  the  daughter-cells  is 
preceded,  if  not  caused,  by  an  inequality  of  the  asters ;  but  we  are 
still  almost  entirely  ignorant  of  the  ultimate  cause  of  this  inequality. 
In  the  cleavage  of  the  animal  ^^^  unequal  division  is  closely  con- 
nected with  the  distribution  of  yolk  —  a  fact  generalized  by  Balfour 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  2/3 

in  the  statement  ('80)  that  the  size  of  the  cells  formed  in  cleavage 
varies  inversely  to  the  relative  amount  of  protoplasm  in  the  region  of 
the  ^g^  from  which  they  arise.  Thus,  in  all  telolecithal  ova,  where 
the  deutoplasm  is  mainly  stored  in  the  lower  6r  vegetative  hemi- 
sphere, as^in  many  worms,  mollusks,  and  vertebrates,  the  cells  of  the 
upper  or  protoplasmic  hemisphere  are  smaller  than  those  of  the  lower, 
and  may  be  distinguished  as  microineres  from  the  larger  macromeres 
of  the  lower  hemisphere.  The  size- ratio  between  micromeres  and 
macromeres  is  on  the  whole  directly  proportional  to  the  ratio  between 
protoplasm  and  deutoplasm.  Partial  or  discoidal  cleavage  occurs 
when  the  mass  of  deutoplasm  is  so  great  as  entirely  to  prevent  cleav- 
age in  the  lower  hemisphere. 


B 

Fig.  124.  —  Partial  or  meroblastic  cleavage  in  the  squid  Loligo.    [Watase.] 

Balfour's  law  undoubtedly  explains  a  large  number  of  cases,  but 
by  no  means  all ;  for  innumerable  cases  are  known  in  which  no  cor- 
relation can  be  made  out  between  the  distribution  of  inert  substance 
and  the  inequality  of  division.  This  is  the  case,  for  example,  with 
the  teloblasts  mentioned  above,  which  contain  no  deutoplasm,  yet 
regularly  divide  unequally.  It  seems  to  be  inapplicable  to  the  in- 
equalities of  the  first  two  divisions  in  annelids  and  gasteropods.  It 
is  conspicuously  inadequate  in  the  history  of  individual  blastomeres, 
where  the  history  of  division  has  been  accurately  determined.  In 
Nereis^  for  example,  a  large  cell  known  as  the  first  somatoblast, 
formed  at  the  fourth  cleavage  (X,  Fig.  122,  E),  undergoes  an  inva- 
riable order  of  division,  three  unequal  divisions  being  followed  by 
an  equal  one,  then  by  three  other  unequal  divisions,  and  again  by  an 
equal.     This  cell  contains  no  deutoplasm  and  undergoes  no  percepti- 


27A 


CELL-DIVISION  AND  DEVELOPMENT 


ble  changes  of  substance.     The  cause  of  the  definite  succession  of 
equal  and  unequal  divisions  is  here  wholly  unexplained. 

Such  cases  prove  that  Balfour's  law  is  only  a  partial  explanation, 
and  is  probably  the  expression  of  a  more  deeply  lying  cause,  and 
there  is  reason  to  believe  that  this  cause  lies  outside  the  immediate 
mechanism   of   mitosis.     Conklin  ('94)  has   called    attention    to   the 


Fig.  125.  —  Embryos  of  the  earthworm  Allolobophora  fxtlda,  showing  teloblasts  or  apical  cells. 
A.  Gastrula  from  the  ventral  side.  B.  The  same  from  the  right  side;  «?,  the  terminal  telo- 
blasts ox  primary  mesoblasfs,  which  bud  forth  the  mesoblast-bands,  cell  by  cell;  /,  lateral  teloblasts, 
comprising  a  neuroblast,  nb,  from  which  the  ventral  nerve-cord  arises,  and  two  nephroblasis,  n,  of 
somewhat  doubtful  nature  but  probably  concerned  in  the  formation  of  the  nephridia.  C.  Lateral 
group  of  teloblasts,  more  enlarged,  the  neuroblast,  nb,  in  division ;  n,  the  nephroblasts.  D.  The 
primary  rnesoblasts  enlarged;  one  in  division. 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  275 

fact  ^  that  the  immediate  cause  of  the  inequality  probably  does  not 
lie  either  in  the  nucleus  or  in  the  amphiaster  ;  for  not  only  the 
chromatin-halves,  but  also  the  asters,  are  exactly  equal  in  the  early 
prophasesr,  and  the  inequality  of  the  asters  only  appears  as  the 
division  proceeds.  Probably,  therefore,  the  cause  lies  in  some  rela- 
tion between  the  mitotic  figure  and  the  cell-body  in  which  it  lies. 
I  believe  there  is  reason  to  accept  the  conclusion  that  this  relation 
is  one  of  position,  however  caused.  A  central  position  of  the  mitotic 
figure  results  in  an  equal  division ;  an  eccentric  position  caused  by  a 
radial  movement  of  the  mitotic  figure,  in  the  direction  of  its  axis 
towards  the  periphery,  leads  to  unequal  division,  and  the  greater  the 
eccentricity,  the  greater  the  inequality,  an  extreme  form  being  beauti- 
fully shown  in  the  formation  of  the  polar  bodies.  Here  the  original 
amphiaster  is  perfectly  symmetrical,  with  the  asters  of  equal  size 
(Fig.  71,  A).  As  the  spindle  rotates  into  its  radial  position  and 
approaches  the  periphery,  the  development  of  the  outer  aster  be- 
comes, as  it  were,  suppressed,  while  the  central  aster  becomes  enor- 
mously large.  The  size  of  tJie  aster,  in  other  words,  depends  upon  the 
extent  of  the  cytoplasmic  area  that  falls  zvithin  the  sphere  of  influence 
of  the  centrosome ;  and  this  area  depends  upon  the  position  of  the 
centrosome.  If,  therefore,  the  polar  amphiaster  could  be  artificially 
prevented  from  moving  to  its  peripheral  position,  the  ^^^  would 
probably  divide   equally. 

The  causes  that  determine  the  position  of  the  amphiaster  are 
scarcely  known.  It  has  been  proved  by  experiment  that  in  some 
cases  this  position  may  be  determined  by  mechanical  causes.  Thus, 
Driesch  has  shown  that  when  the  eggs  of  sea-urchins  are  flattened 
by  pressure,  the  amphiasters  all  assume  the  position  of  least  resist- 
ance, i.e.  parallel  to  the  flattened  sides,  so  that  the  cleavages  are  all 
vertical,  and  the  <tgg  segments  as  a  flat  plate  of  eight,  sixteen,  or 
thirty-two  cells  (Fig.  ^35).  This  is  totally  different  from  the  normal 
iorm  of  cleavage  ;  yet  such  eggs,  when  released  from  pressure,  are 
capable  of  development  and  give  rise  to  normal  embryos.  This 
interesting  experiment  makes  it  highly  probable  that  the  disc-like 
cleavage  of  meroblastic  eggs,  like  that  of  the  squid  or  bird,  is  a 
mechanical  result  of  the  accumulation  of  yolk  by  which  the  forma- 
tive protoplasmic  region  of  the  ovum  is  reduced  to  a  thin  layer  at 
the  upper  pole  ;  and  it  indicates,  further,  that  the  unequal  cleavage 
of  less  modified  telolecithal  eggs,  like  those  of  the  frog  or  snail,  are 
in  like  manner  due  to  the  displacement  of  the  mitotic  figures  towards 
the  upper  pole.  Even  here,  however,  the  hypothesis  of  a  merely 
mechanical    displacement    probably  does  not  touch  the  root  of  the 

^  In  the  cleavage  of  gasteropod  eggs. 


2/6  CELL-DIVISION  AND  DEVELOPMENT 

matter;  for  it  will  not  account  for  the  eccentric  position  of  the 
spindle  in  the  formation  of  the  polar  bodies  or  in  teloblasts.  Neither 
will  it  explain  the  eccentric  position  of  the  horizontal  spindle  in  such 
cases  as  the  first  cleavage  of  the  annelid  ^^^g.  In  Nereis,  for  exam- 
ple (Figs.  43,  122),  the  inequality  of  the  first  cleavage  is  predeter- 
mined long  before  actual  division  both  by  an  eccentric  position  of 
the  spindle  and  an  inequality  in  the  asters,  neither  of  which  can  be 
referred  to  an  unequal  horizontal  distribution  of  the  yolk.^  In  this 
and  many  similar  cases  we  must  assume  more  subtle  causes  lying 
in  the  organization  of  the  cytoplasmic  mass,  or  rather  of  the  Q,g'g 
as  a  whole ;  but  these  deeper  causes  still  lie  beyond  our  grasp. 
Unequal  division,  which  plays  so  important  a  part  in  development, 
still  therefore  awaits  a  final  explanation,  and  until  this  is  forthcom- 
ing we  have  but  a  vague  comprehension  of  the  primary  factors  of 
growth. 

Hertwig's  Development  of  Sachs's  Laiv. — We  have  now  to  consider 
two  additional  laws  of  cell-division  formulated  by  Oscar  Hertwig  in 
1884,  which  bear  directly  on  the  facts  just  outlined  and  which  lie 
behind  Sachs's  principle  of  the  rectangular  intersection  of  successive 
division-planes.     These  are :  — 

1 .  The  nucleus  tends  to  take  up  a  position  at  the  centre  of  its  sphere 
of  influence,  i.e.  of  the  protoplasmic  mass  in  which  it  lies. 

2.  The  axis  of  the  mitotic  figures  typically  lies  in  the  longest  axis 
of  the  protoplasmic  mass,  and  division  tJierefore  tends  to  cut  this  axis 
at  a  right  angle. 

The  second  law  explains  not  only  the  mode  of  division  in  flattened 
eggs,  but  also  the  normal  succession  of  the  division-planes  according 
to  Sachs's  second  law.  The  first  division  of  a  homogeneous  spherical 
Qgg,  for  example,  is  followed  by  a  second  division  at  right  angles  to  it, 
since  each  hemisphere  is  twice  as  long  in  the  plane  of  division  as  in 
any  plane  vertical  to  it.  The  mitotic  figure  of  the  second  division 
lies  therefore  parallel  to  the  first  plane,  which  forms  the  base  of  the 
hemisphere,  and  the  ensuing  division  is  vertical  to  it.  The  same 
applies  to  the  third  division,  since  each  quadrant  is  as  long  as  the 
entire  Qgg  while  at  most  only  half  its  diameter.  Division  is  there- 
fore transverse  to  the  long  axis  and  vertical  to  the  first  two  planes. 

Hertwig's  second  law  has  caused  much  discussion  and  has  been 
shown  to  have  many  exceptions,  as  for  instance  in  the  cambium-cells 
of  plants  and  in  columnar  epithelium.  While  undoubtedly  one  of 
the  most  important  laws  of  cell-division  thus  far  determined,  it  only 
pushes  the  analysis   a   stage  further  back,  and  leaves   unexplained 

^  In  an  earlier  paper  I  made  the  erroneous  statement  that  the  first  cleavage-spindle  of 
Nereis  lies  centrally  in  the  egg.  Later  and  more  careful  studies  by  means  of  sections  prove 
that  this  was  incorrect. 


GEOMETRICAL   RELATIONS   OF  CLEAVAGE-FORMS  2// 

the  nature  of  the  forces  that  determme  the  position  of  the  spindle- 
axis.  Pfliiger^  assumed  that  this  position  must  be  that  of  least 
resistance  to  the  elongation  of  the  spindle,  which  is  obviously  in  the 
long  axisyof  the  protoplasmic  mass;  and  the  same  view  has  been 
advocated  by  Braem  and  Driesch.  Now,  there  can  of  course  be  no 
doubt  that  the  final  direction  of  the  spindle,  like  that  of  any  body,  is 
the  position  of  least  resistance,  i.e.  the  position  of  equilibrium  de- 
termined by  the  resultant  of  all  the  forces  operating  upon  it.  The 
undetermined  point  is  whether  these  forces  are  of  a  simple  mechani- 
cal nature,  such  as  pressure  and  the  like,  or  of  a  more  subtle  physio- 
logical character.  Roux  seeks  them  in  the  "tractive  forces"  of  the 
protoplasmic  mass  modified  by  an  innate  predisposition  to  a  partic- 
ular form  and  succession  of  divisions  that  has  its  seat  in  the  nucleus. 
Heidenhain  identifies  them  with  conditions  of  intra-cellular  tension 
determined  by  the  astral  rays. 

It  cannot  be  doubted  that  all  these  forces  may  play  a  part  in  de- 
termining the  position  of  the  spindle,  but  it  must  be  confessed  that 
the  problem  is  still  very  far  from  a  solution.  In  some  cases  Hert- 
wig's  law  is  directly  opposed  to  the  facts,  the  spindle  lying  trans- 
versely to  the  axis  of  the  protoplasmic  mass.  In  other  cases,  as  for 
instance  in  the  division  of  some  Protozoa  {EuglypJia,  t.  Schewiakoff) 
and  in  segmenting  ova  {Crepidtila,  t.  Conklin),  the  protoplasmic 
elongation  leads  the  way,  and  may  be  fully  determined  before  the 
spindle  is  formed.  In  still  other  cases  the  reverse  is  true,  as  in  the 
formation  of  the  polar  bodies,  where  the  spindle  forms  and  rotates 
into  position  before  the  Qgg  shows  any  corresponding  change  of 
form.  In  many  ova  we  can  assign  no  mechanical  cause  for  the  rota- 
tion, such  as  the  pressure  of  deutoplasm  and  the  like ;  and  even 
when  deutoplasm  is  present,  its  position  is  such  that  we  should 
expect  a  horizontal  rather  than  a  vertical  position  of  the  polar 
spindles  were  it  a  mechanical  result  of  the  presence  of  deutoplasm. 

The  ultimate  determination  of  the  planes  of  division  is  probably  to 
be  sought  in  those  influences  that  determine  the  movements  of  the 
centrosomes.  Sachs's  law  of  rectangular  succession  is  primarily  a 
result  of  the  fact  that  the  daughter-centrosomes  typically  diverge, 
and  so  determine  the  spindle-axis,  in  a  line  which  is  at  right  angles 
to  the  axis  of  the  mother-spindle ;  hence  the  ensuing  cleavage  is 
vertical  to  the  last.^  What  we  do  not  really  understand  is  the  prin- 
ciple by  which  this  typical  succession  is  modified.  The  pressure- 
experiments  prove  that  the  modifications  may  be  produced  by  simple 

^  '84,  p.  613. 

-  In  this  we  find  also  an  explanation  of  the  fact  first  observed  by  Roux  in  the  frog's  egg, 
and  confirmed  by  me  in  the  sea-urchin  egg,  that  the  first  plane  of  cleavage  passes  through 
the  sperm-track  and  hence  approximately  through  the  entrance-point  of  the  spermatozoon. 


2/8  CELL-DIVISION  AND  DEVELOPMENT 

mechanical  means.  The  history  of  division  in  the  cambium-cells 
and  columnar  epithelium  seems  to  show  that  neither  direct  pressure 
nor  the  shape  of  the  cells  caused  by  it  can  be  the  ultimate  cause. 
The  succession  of  divisions,  always  in  the  same  plane,  in  apical  cells 
and  in  teloblasts,  is  directly  related  with  a  deeply  lying  law  of  growth 
that  affects  the  whole  developing  organism,  and  we  cannot  at  pres- 
ent distinguish  in  such  cases  between  cause  and  effect ;  for  whether 
the  apical  growth  of  the  body  as  a  whole  is  caused  by  local  condi- 
tions within  the  apical  cells,  or  the  reverse,  is  undetermined.  This 
unsatisfactory  result  shows  how  far  we  still  are  from  an  understand- 
ing of  the  fundamental  laws  of  growth  and  their  relation  to  cell- 
division,  and  how  vast  a  field  for  experimental  research  lies  open  in 
this  direction. 

B.     Promorphological  Relations  of  Cleavage 

The  cleavage  of  the  ovum  has  thus  far  been  considered  merely  as 
a  problem  of  cell-division.  We  have  now  to  regard  it  in  a  far  more 
interesting  and  suggestive  aspect ;  namely,  in  its  morphological  rela- 
tions to  the  body  to  which  it  gives  rise.  From  what  has  been  said 
thus  far  it  might  be  supposed  that  the  ^^g  simply  splits  up  into  indif- 
ferent cells  which,  to  use  the  phrase  of  Pfluger,  have  no  more  definite 
relation  to  the  structure  of  the  adult  body  than  have  snow-flakes  to  the 
avalanche  to  which  they  contribute.  Such  a  conclusion  would  be 
totally  erroneous.  It  is  a  remarkable  fact  that  in  a  very  large  num- 
ber of  cases  a  precise  relation  exists  between  the  cleavage-products 
and  the  adult  parts  to  which  they  give  rise  ;  and  this  relation  may 
often  be  traced  back  to  the  beginning  of  development,  so  that  from 
the  first  division  onwards  we  are  able  to  predict  the  exact  future  of 
every  individual  cell.  In  this  regard  the  cleavage  of  the  ovum  often 
goes  forward  with  a  wonderful  clock-like  precision,  giving  the  impres- 
sion of  a  strictly  ordered  series  in  which  every  division  plays  a  defi- 
nite role  and  has  a  fixed  relation  to  all  that  precedes  and  follows  it. 

But  more  than  this,  the  apparent  predetermination  of  the  embryo 
may  often  be  traced  still  further  back  to  the  regions  of  the  undivided 
and  even  unfertilized  ovum.  The  ^gg,  therefore,  may  exhibit  a  dis- 
tinct promorphology ;  and  the  morphological  aspect  of  cleavage 
must  be  considered  in  relation  to  the  promorphology  of  the  ovum 
of  which  it  is  an  expression. 

I.    PromorpJiology  of  tJic  Ovum 

{a)  Polarity  and  the  Egg;-axis.  —  It  was  long  ago  recognized  by 
von  Baer  ('34)  that  the  unsegmented  egg  of  the  frog  has  a  definite 
c'gg-axjs  connecting  two  differentiated  poles,  and  that  the  position  of 


PROMORPIIOLOGICAL  RELATIONS   OF  CLEAVAGE  2/9 

the  embryo  is  definitely  related  to  it.  The  great  embryologist 
pointed  out,  further,  that  the  early  cleavage-planes  also  are  defi- 
nitely related  to  it,  the  first  two  passing  through  it  in  two  meridians 
intersecting  each  other  at  a  right  angle,  while  the  third  is  transverse 
to  it,  and  is  hence  equatorial.^  Remak  afterwards  recognized  the 
fact'-^  that  the  larger  cells  of  the  lower  hemisphere  represent,  broadly 
speaking,  the  "vegetative  layer"  of  von  Baer,  i.e.  the  inner  germ- 
layer  or  entoblast,  from  which  the  alimentary  organs  arise ;  while 
the  smaller  cells  of  the  upper  hemisphere  represent  the  ''  animal 
layer,"  outer  germ-layer  or  ectoblast  from  which  arise  the  epidermis, 
the  nervous  system,  and  the  sense-organs.  This  fact,  afterwards 
confirmed  in  a  very  large  number  of  animals,  led  to  the  designation 
of  the  two  poles  as  animal  and  vegetative,  formative  and  nutritive,  or 
protoplasmic  and  dentoplasmic,  the  latter  terms  referring  to  the  fact 
that  the  nutritive  deutoplasm.  is  mainly  stored  in  the  lower  hemi- 
sphere, and  that  development  is  therefore  more  active  in  the  upper. 
The  polarity  of  the  ovum  is  accentuated  by  other  correlated  phe- 
nomena. In  every  case  where  an  egg-axis  can  be  determined  by  the 
accumulation  of  deutoplasm  in  the  lower  hemisphere  the  egg-nucleus 
sooner  or  later  lies  eccentrically  in  the  upper  hemisphere,  and  the 
polar  bodies  are  formed  at  the  upper  pole.  Even  in  cases  where 
the  deutoplasm  is  equally  distributed  or  is  wanting  —  if  there  really 
be  such  cases  —  an  egg-axis  is  still  determined  by  the  eccentricity 
of  the  nucleus  and  the  corresponding  point  at  which  the  polar  bodies 
are  formed. 

In  vastly  the  greater  number  of  cases  the  polarity  of  the  ovum  has 
a  definite  promorphological  significance ;  for  the  egg-axis  shows  a 
definite  and  constant  relation  to  the  axes  of  the  adult  body.  This 
relation  is,  it  is  true,  somewhat  variable  in  different  animals,  yet  the 
evidence  indicates  that  as  a  rule  it  is  constant  in  a  given  species.  It 
is  a  very  general  rule-  that  the  upper  pole,  as  marked  by  the  posi- 
tion of  the  polar  bodies,  lies  in  the  median  plane  at  a  point  which  is 
afterwards  found  to  lie  at  or  near  the  anterior  end.  Throughout 
the  annelids  and  mollusks,  for  example,  the  upper  pole  is  the  point 
at  which  the  cerebral  ganglia  are  afterwards  formed ;  and  these 
organs  lie  in  the  adult  on  the  dorsal  side  near  the  anterior  extremity. 
This  relation  holds  true  for  many  of  the  Bilateralia,  though  the 
primitive  relation  is  often  disguised  by  asymmetrical  growth  in  the 
later  stages,  such  as  occur  in  echinoderms.  It  is  not,  however,  a 
universal  rule.  The  recent  observations  of  Castle  ('96),  which  are 
in  accordance  with  the    earlier  work  of  Seeliger,  show  that  in  the 

^  The  third  plane  is  in  this  case  not  precisely  at  the  equator,  but  considerably  above  it, 
forming  a  "  parallel ''  cleavage. 

-'55'  p-  ';^<^- 


28o  CELL-DIVISION  AND  DEVELOPMENT 

tunicate  Ciona  the  usual  relation  is  reversed,  the  polar  bodies  being 
formed  at  the  vegetative  {i.e.  deutoplasmic)  pole,  which  afterwards 
becomes  the  ventral  side  of  the  larva.  In  Ascaris  Boveri's  observa- 
tions seem  to  show  that  the  position  of  the  polar  bodies  has  no  con- 
stant relation  to  the  adult  axes,  and  Hacker  describes  a  similar  vari- 
ability in  the  copepods  (Fig.  130).  My  own  observations  on  the 
echinoderm-Qgg  indicate  that  here  also  the  primitive  egg-axis  has 
an  entirely  inconstant  and  casual  relation  to  the  gastrula-axis.  It 
may  perhaps  still  be  possible  to  show  that  these  exceptions  are  only 
apparent,  and  the  principle  involved  is  too  important  to  be  accepted 
without  further  proof.  As  the  facts  now  stand,  however,  they  seem 
to  admit  of  no  other  conclusion  than  that  the  relation  of  the  primitive 
egg-axis  to  the  adult  axes  is  not  absolutely  constant,  and  may  in  par- 
ticular cases  be  variable.  To  admit  this  is  to  grant  that  this  relation 
is  not  of  a  fundamental  character,  and  that  the  axes  of  the  adult  are 
not  predetermined  from  the  beginning,  but  are  established  in  the  ^gg 
in  the  course  of  development. 

{b)  Axial  Relations  of  the  Primary  Cleavage-planes.  —  Since  the 
egg-axis  is  definitely  related  to  the  embryonic  axes,  and  since  the 
first  two  cleavage-planes  pass  through  it,  we  may  naturally  look  for  a 
definite  relation  between  these  planes  and  the  embryonic  axes ;  and 
if  such  a  relation  exists,  then  the  first  two  or  four  blastomeres  must 
likewise  have  a  definite  prospective  value  in  the  development.  Such 
relations  have,  in  fact,  been  accurately  determined  in  a  large  number 
of  cases.  The  first  to  call  attention  to  such  a  relation  seems  to  have 
been  Newport  ('54),  who  discovered  the  remarkable  fact  that  tJie  first 
cleavage-plane  in  the  frogs  egg  coincides  with  the  median  plane  of  the 
adult  body ;  that,  in  other  words,  one  of  the  first  two  blastomeres 
gives  rise  to  the  left  side  of  the  body,  the  other  to  the  right.  This 
discovery,  though  long  overlooked  and,  indeed,  forgotten,  was  con- 
firmed more  than  thirty  years  later  by  Pfliiger  and  Roux  ('87).  It 
was  placed  beyond  all  question  by  a  remarkable  experiment  by  Roux 
('88),  who  succeeded  in  killing  one  of  the  blastomeres  by  puncture 
with  a  heated  needle,  whereupon  the  uninjured  cell  gave  rise  to  a 
half-body  as  if  the  embryo  had  been  bisected  down  the  middle  line 
(Fig.  131). 

A  similar  result  has  been  reached  in  a  number  of  other  animals  by 
following  out  the  cell-lineage ;  e.g.  by  Van  Beneden  and  Julin  ('84) 
in  the  ^g^  of  the  tunicate  Claveliiia  (Fig.  126),  and  by  Watase  ('91) 
in  the  eggs  of  cephalopods  (Fig.  127).  In  both  these  cases  all  the 
early  stages  of  cleavage  show  a  beautiful  bilateral  symmetry,  and  not 
only  can  the  right  and  left  halves  of  the  segmenting  Q,gg  be  distin- 
guished with  the  greatest  clearness,  but  also  the  anterior  and  poste- 
rior regions,  and  the  dorsal  and  ventral  aspects.     These  discoveries 


PROMORPHOLOGICAL   RELATIONS   OF  CLEAVAGE 


281 


seemed,  at  first,  to  justify  the  hope  that  a  fundamental  law  of  develop- 
ment had  been  discovered,  and  Van  Beneden  was  thus  led,  as  early 
as  1883,  to  express  the  view  that  the  development  of  all  bilateral 
animals  wguld  probably  be  found  to  agree  with  the  frog  and  ascidian 
in  respect  to  the  relations  of  the  first  cleavage. 

This  conclusion  was  soon  proved  to  have  been  premature.     In  one 
series  of  forms,  not  the  first  but  the  second  cleavage-plane  was  found 


Fig.  126.  —  Bilateral  cleavage  of  the  tunicate  egg, 
A,  Four-celled  stage  of  Clavelina,  viewed  from  the  ventral  side.    B.  Sixteen-cell  stage  (Van 
Beneden  and  Julin).      C.  Cross-section  through  the  gastrula  stage  (Castle);   a,  anterior; 
/,  posterior  end ;  /,  left,  r,  right  side.     [Orientation  according  to  CASTLE.] 


to  coincide  with  the  future  long  axis  {Nereis,  and  some  other  annelids; 
Ci'epidida,  Umbrella,  and  other  gasteropods).  In  another  series  of 
forms  neither  of  the  first  cleavages  passes  through  the  median  plane, 
but  both  form  an  angle  of  about  45°  to  it  {Clepsine  and  other  leeches ; 
Rhynchelmis  and  other  annelids ;  Planorbis,  Nassa,  Unto,  and  other 
mollusks ;  Discoccelis  and  other  platodes).  In  a  few  cases  the  first 
cleavage  departs  entirely  from  the  rule,  and  is  equatorial,  as  in  Ascaris 
and  some   other  nematodes.     The  whole  subject  was  finally  thrown 


282 


CELL-DIVISION  AND  DEVELOFMENT 


into  apparent  confusion,  first  by  the  discovery  of  Clapp  ('91),  Jordan, 
and  Eycleshymer  (94)  that  in  some  cases  there  seems  to  be  no  con- 
stant relation  whatever  between  the  early  cleavage-planes  and  the 
adult  axes,  even  in  the  same  species  (teleosts,  urodeles)  ;  and  even  in 
the  frog  Hertwig  showed  that  the  relation  described  by  Newport  and 
Roux  is  not  invariable.  Driesch  finally  demonstrated  that  the  direc- 
tion of  the  early  cleavage-planes  might  be  artificially  modified  by 
pressure  without  perceptibly  affecting  the  end-result  (cf.  p.  309). 

These  facts  prove  that  the  promorphology  of  the  early  cleavage- 
forms  can  have  no  fundamental  significance.  Nevertheless,  they  are 
of  the  highest  interest  and  importance  ;  for  the  fact  that  the  forma- 


:.-'  r 


P 


Fig.  127.  —  Bilateral  cleavage  of  the  squid's  egg.     [Watase.] 
A.   Eight-cell  stage.      B.  The  fifth  cleavage  in  progress.     The  first  cleavage  {a-p)  coincides 
with  the  future  median  plane ;  the  second  (/-/-)  is  transverse. 


tive  forces  by  which  development  is  determined  may  or  may  not 
coincide  with  those  controlling  the  cleavage,  gives  us  some  hope  of 
disentangHng  the  complicated  factors  of  development  through  a  com- 
parative study  of  the  different  forms. 

if)  OtJiei'  P romorpJioIogical  CJiaractcrs  of  the  Ovum.  —  Besides  the 
polarity  of  the  ovum,  which  is  the  most  constant  and  clearly  marked 
of  its  promorphological  features,  we  are  often  able  to  discover  other 
characters  that  more  or  less  clearly  foreshadow  the  later  develop- 
ment. One  of  the  most  interesting  and  clearly  marked  of  these  is 
the  bilateral  symmetry  of  the  ovum  in  bilateral  animals,  which  is 
sometimes  so  clearly  marked  that  the  exact  position  of  the  embryo 
may  be  predicted  in  the  unfertilized  o-^'g,  sometimes  even  before  it  is 
laid.     This  is  the  case,  for  example,  in  the  cephaloj)od  ^•^Ji.'g,  as  shown 


PKOMORPHOLOGICAL   RELATIONS   OF  CLEAVAGE 


283 


by  Watase  (Fig.  128).  Here  the  form  of  the  new-laid  ^^g^  before 
cleavage  begins,  distinctly  foreshadows  that  of  the  embryonic  body, 
and -forms  as  it  were  a  mould  in  which  the  whole  development  is  cast. 
Its  general  shape  is  that  of  a  hen's  ^gg  slightly  flattened  on  one  side, 
the  narrow  end,  according  to  Watase,  representing  the  dorsal  aspect, 
the  broad  end  the  ventral  aspect,  the  flattened  side  the  posterior 
region,  and  the  more  convex  side  the  anterior  region.  All  tJie  early 
cleavage-fiirroivs  are  bilaterally  a^'ranged  ivith  respect  to  the  plane  of 
symmetry  in  the  undivided  egg ;  and  the  same  is  true  of  the  later 
development  of  all  the  bilateral  parts. 


/ 


Fig.  128.  —  Outline  of  unsegmented  squid's  ^gg,  to  show  bilaterality.     [Watase.] 
A.  From  right  side.     B.  From  the  posterior  aspect. 
a-p,  antero-posterior  axis ;  d-v,  dorso-ventral  axis ;  /,  left  side ;  r,  right  side. 

Scarcely  less  striking  is  the  case  of  the  insect  ^gg,  as  has  been 
pointed  out  especially  by  Hallez,  Blochmann,  and  Wheeler  (Figs. 
44,  129).  In  a  large  number  of  cases  the  ^gg  is  elongated  and 
bilaterally  symmetrical,  and,  according  to  Blochmann  and  Wheeler, 
may  even  show  a  bilateral  distribution  of  the  yolk  corresponding 
with  the  bilaterality  of  the  ovum.  Hallez  asserts  as  the  result  of  a 
study  of  the  cockroach  {Peripla7teta),  the  water-beetle  {Hydrophihts), 
and  the  locust  {Locnsta)  that  "  the  egg-cell  possesses  the-  same  orienta- 
tion as  the  maternal  organism  that  produces  it ;  it  has  a  cephalic 
pole  and  a  caudal  pole,  a  right  side  and  a  left,  a  dorsal  aspect  and  a 
ventral  ;  and  these  different  aspects  of  the  egg-cell  coincide  with  the 
corresponding  aspects  of  the  embryo."^  Wheeler  ('93),  after  ex- 
amining some  thirty  different  species  of   insects,  reached  the  same 

1  See  Wheeler,  '93,  p.  67. 


:84 


CELL-DIVISION  AND  DEVELOPMENT 


result,  and  concluded  that  even  when  the  (^gg  approaches  the 
spherical  form  the  symmetry  still  exists,  though  obscured.  More- 
over, according  to  Hallez  ('86)  and  later  writers,  the  ^gg  always  lies 
in  the  same  position  in  the  oviduct,  its  cephalic  end  being  turned 


(t 


Fig.  129.  — Eggs  of  the  insect  Corixa.     [Metschnikoff.] 
A.  Early  stage  before  formation  of  the  embryo,  from  one  side.      B.  The  same  viewed  in  the 
plane  of  symmetry.     C.  The  embryo  in  its  final  position. 

a,  anterior  end;  p,  posterior;  /,  left  side,  r,  right;  v,  ventral,  d,  dorsal  aspect.  (These  letters 
refer  to  the  final  position  of  the  embryo,  which  is  nearly  diametrically  opposite  to  that  in  which  it 
first  develops)  ;  ?«,  micropyle ;  near/  is  the  pedicle  by  which  the  egg  is  attached. 

forwards  towards  the  upper  end  of  the  oviduct,  and  hence  towards 
the  head-end  of  the  mother.^ 


'  The  micropyle  usually  lies  at  or  near  the  anterior  end,  but  may  l)e  at  the  posterior. 
It  is  a  very  important  fact  that  the  position  of  the  polar  bodies  varies,  being  sometimes  at 
the  anterior  end,  sometimes  on  the  side,  either  dorsal  or  lateral  (Ileider,  Blochmann). 


PKOMORPHOLOGICAL   RELATIONS   GF  CLEAVAGE  285 


2.    Meaning  of  the  Proinorphology  of  the  Ovum 

The  interpretation  of  the  promorphology  of  the  ovum  cannot  be 
adequately  treated  apart  from  the  general  discussion  of  development 
given  in  the  following  chapter ;  nevertheless  it  may  conveniently  be 
considered  at  this  point.  Two  fundamentally  different  interpreta- 
tions of  the  facts  have  been  given.  On  the  one  hand,  it  has  been 
suggested  by  Flemming  and  Van  Beneden/  and  urged  especially 
by  Whitman,^  that  the  cytoplasm  of  the  ovum  possesses  a  definite 
primordial  organization  which  exists  from  the  beginning  of  its  exis- 
tence even  though  invisible,  and  is  revealed  to  observation  through 
polar  differentiation,  bilateral  symmetry,  and  other  obvious  characters 
in  the  unsegmented  Qgg.  On  the  other  hand,  it  has  been  maintained 
by  Pfliiger,  Mark,  Oscar  Hertwig,  Driesch,  Watase,  and  the  writer 
that  all  the  promorphological  features  of  the  ovum  are  of  secondary 
origin;  that  the  egg-cytoplasm  is  at  the  beginning  isotropous  —  i.e. 
indifferent  or  homaxial  —  and  gradually  acquires  its  promorphological 
features  during  its  pre-embryonic  history.  Thus  the  Qgg  of  a  bilateral 
animal  is  not  at  the  beginning  actually,  but  only  potentially,  bilateral. 
Bilaterality  once  established,  however,  it  forms  as  it  were  the  mould 
in  which  the  cleavage  and  other  operations  of  development  are  cast. 

I  believe  that  the  evidence  at  our  command  weighs  heavily  on 
the  side  of  the  second  view,  and  that  the  first  hypothesis  fails  to 
take  sufficient  account  of  the  fact  that  development  does  not  nec- 
essarily begin  with  fertilization  or  cleavage,  but  may  begin  at  a  far 
earlier  period  during  ovarian  life.  As  far  as  the  visible  promorpho- 
logical features  of  the  ovum  are  concerned,  this  conclusion  is  beyond 
question.  The  only  question  that  has  any  meaning  is  whether  these 
visible  characters  are  merely  the  expression  of  a  more  subtle  pre- 
existing invisible  organization  of  the  same  kind.  I  do  not  believe 
that  this  question  can  be  answered  in  the  affirmative  save  by  the 
trite  and,  from  this  point  of  view,  barren  statement  that  every  effect 
must  have  its  pre-existing  cause.  That  the  Qgg  possesses  no  fixed 
and  predetermined  cytoplasmic  localization  with  reference  to  the 
adult  parts,  has,  I  think,  been  demonstrated  through  the  remarkable 
experiments  of  Driesch,  Roux,  and  Boveri,  which  show  that  a  frag- 
ment of  the  Qgg  may  give  rise  to  a  complete  larva  (p.  308).  There 
is  strong  evidence,  moreover,  that  the  egg-axis  is  not  primordial,  but 
is  established  at  a  particular  period ;  and  even  after  its  establishment 
it  may  be  entirely  altered  by  new  conditions.  This  is  proved,  for 
example,  by  the  case  of  the  frog's  Qgg,  in  which,  as  Pfliiger  ('84), 
Born  ('85),  and  Schultze  ('94)  have"  shown,  the  cytoplasmic  materials 

1  See  p.  298.  2  cf_  pp,  299,  300. 


260 


CELL-DIVISION  AND  DEVELOPMENT 


may   be  entirely  re-arranged  under  the  influence  of  gravity,   md  a 
axis  established.      In   sea-urchins,  my   own    observations   ('95) 


new 


render  it  probable  that  the  egg-axis  is  not  finally  established  until 
after  fertilization.  Finally,  it  is  becoming  more  and  more  doubt- 
ful whether  the  relation  of  the  egg-axis  to  the  adult  axis  has  so  deep 
a  significance  as  was  at  first  assumed.     This  relation  has  been  found 


Fig.  130.  —  Variations  in  the  axial  relations  of  the  eggs  of  Cyclops.  From  sections  of  the  eggs 
as  tliey  lie  in  the  oviduct.     [Hacker.] 

A.  Group  of  eggs  showing  variations  in  relative  position  of  the  polar  spindles  and  the  sperm- 
nucleus  (the  latter  black) ;  in  a  the  sperm-nucleus  is  opposite  to  the  polar  spindle,  in  b,  near  it  or 
at  the  side.  B.  Group  showing  variations  in  the  axis  of  first  cleavage  with  reference  to  the  polar 
bodies  (the  latter  black)  ;  a,  b,  and  c,  show  three  different  positions. 


to  vary  not  only  in  nearly  related  forms  (insects),  but  even  in  the 
same  species  {Ascaris,  according  to  Boveri  and  others  ;  Toxopiicustcs, 
according  to  my  own  observations;  copepods,  according  to  Hacker). 
All  these  and  many  other  similar  facts  force  us,  I  think,  to  the 
conclusion  that  the  promorphological  features  of  the  o^^^  are  as  truly 
a  result  of  development  as  the  characters  coming  into  view  at  later 


PROMORPHOLOGICAL   RELATIONS   OF  CLEAVAGE  287 

Stages.  They  are  gradually  established  during  the  pre-embryonic 
stages,  and  the  ^^^,  when  ready  for  fertilization,  has  already  accom- 
plished part  of  its  task  by  laying  the  basis  for  what  is  to  come. 

Mark,  who  was  one  of  the  first  to  examine  this  subject  carefully, 
concluded^' that  the  ovum  is  at  first  an  indifferent  or  homaxial  cell 
{i.e.  isotropic),  which  afterwards  acquires  polarity  and  other  promor- 
phological  features.^  The  same  view  was  very  precisely  formulated 
by  Watase  in  1 891,  in  the  following  statement,  which  I  believe  to 
express  accurately  the  truth  :  "  It  appears  to  me  admissible  to  say 
at  present  that  the  ovum,  which  may  start  out  without  any  definite 
axis  at  first,  may  acquire  it  later,  and  at  the  moment  ready  for  its 
cleavage  the  distribution  of  its  protoplasmic  substances  may  be  such 
as  to  exhibit  a  perfect  symmetry,  and  the  furrows  of  cleavage  may 
have  a  certain  definite  relation  to  the  inherent  arrangement  of  the 
protoplasmic  substances  v/hich  constitute  the  ovum.  Hence,  in  a 
certain  case,  the  plane  of  the  first  cleavage-furrow  may  coincide  with 
the  plane  of  the  median  axis  of  the  embryo,  and  the  sundering  of 
the  protoplasmic  material  may  take  place  into  right  and  left,  accord- 
ing to  the  pre-existing  organization  of  the  ^^g  at  the  time  of  cleav- 
age ;  and  in  another  case  the  first  cleavage  may  roughly  correspond 
to  the  differentiation  of  the  ectoderm  and  the  entoderm,  also  accord- 
ing to  the  pre-organized  constitution  of  the  protoplasmic  materials  of 
the  ovum. 

*'  It  does  not  appear  strange,  therefore,  that  we  may  detect  a  cer- 
tain structural  differentiation  in  the  unsegmented  ovum,  with  all  the 
axes  foreshadowed  in  it,  and  the  axial  symmetry  of  the  embryonic 
organism  identical  with  that  of  the  adult."  ^ 

This  passage  contains,  I  believe,  the  gist  of  the  whole  matter,  as 
far  as  the  promorphological  relations  of  the  ovum  and  of  cleavage- 
forms  are  concerned,  though  Watase  does  not  enter  into  the  question 
as  to  how  the  arrangement  of  protoplasmic  materials  is  effected.  In 
considering  this  question,  we  must  hold  fast  to  the  fundamental .  fact 
that  the  ^gg  is  a  cell,  like  other  cells,  and  that  from  an  a  priori  point 
of  view  there  is  every  reason  to  believe  that  the  cytoplasmic  differ- 
entiations that  it  undergoes  must  arise  in  essentially  the  same  way  as 
in  other  cells.  We  know  that  such  differentiations,  whether  in  form 
or  in  internal  structure,  show  a  definite  relation  to  the  environment 
of  the  cell  —  to  its  fellows,  to  the  source  of  food,  and  the  like.  We 
know  further,  as  Korschelt  especially  has  pointed  out,  that  the  egg- 
axis,  as  expressed  by  the  eccentricity  of  the  germinal  vesicle,  often 
shozvs  a  definite  relatioji  to  the  ovarian  tissues,  the  germinal  vesicle 
lying  near  the  point  of  attachment  or  of  food-supply.      Mark  made 

1  '81,  p.  512.  ^  '91,  p.  280. 


288  CELL-DIVISION  AND  DEVELOPMENT 

the  pregnant  suggestion,  in  1881,  that  the  primary  polarity  of  the  ^gg 
might  be  determined  by  "  the  topographical  relation  of  the  egg  (when 
still  in  an  indifferent  state)  to  the  remaining  cells  of  the  maternal  tis- 
sue from  which  it  is  differentiated^'  and  added  that  this  relation  might 
operate  through  the  nutrition  of  the  ovum.  ''  It  would  certainly  be 
interesting  to  know  if  that  phase  of  polar  differentiation  which  is 
manifest  in  the  position  of  the  nutritive  substance  and  of  the  germi- 
nal vesicle  bears  a  constant  relation  to  the  free  surface  of  the  epithe- 
lium from  which  the  ^gg  takes  its  origin.  If,  in  cases  where  the  ^gg 
is  directly  developed  from  epithelial  cells,  this  relationship  were 
demonstrable,  it  would  be  fair  to  infer  the  existence  of  correspond- 
ing, though  obscured,  relations  in  those  cases  where  (as,  for  example, 
in  mammals)  the  origin  of  the  ovum  is  less  directly  traceable  to  an 
epithelial  surface."  ^  The  polarity  of  the  ^gg  would  therefore  be 
comparable  to  the  polarity  of  epithelial  or  gland  cells,  where,  as 
pointed  out  at  p.  40,  the  nucleus  usually  lies  towards  the  base  of  the 
cell,  near  the  source  of  food,  while  the  characteristic  cytoplasmic 
products,  such  as  zymogen  granules  and  other  secretions,  appear  in 
the  outer  portion.^  The  exact  conditions  under  which  the  ovarian 
^gg  develops  are  still  too  little  known  to  allow  of  a  positive  conclu- 
sion regarding  Mark's  suggestion.  Moreover,  the  force  of  Korschelt's 
observation  is  weakened  by  the  fact  that  in  many  eggs  of  the  extreme 
telolecithal  type,  where  the  polarity  is  very  marked,  the  germinal 
vesicle  occupies  a  central  or  sub-central  position  during  the  period  of 
yolk-formation  and  only  moves  towards  the  periphery  near  the  time 
of  maturation. 

Indeed,  in  moUusks,  annelids,  and  many  other  cases,  the  germinal 
vesicle  remains  in  a  central  position,  surrounded  by  yolk  on  all  sides, 
until  the  spermatozoon  enters.  Only  then  does  the  egg-nucleus  move 
to  the  periphery,  the  deutoplasm  become  massed  at  one  pole,  and 
the  polarity  of  the  Qgg  come  into  view  {Nereis,  Figs.  43  and  71).^  In 
such  cases  the  axis  of  the  ^gg  is  not  improbably  predetermined  by 
the  position  of  the  centrosome,  and  we  have  still  to  seek  the  causes 
by  which  the  position  is  established  in  the  ovarian  history  of  the  Q,gg. 
These  considerations  show  that  the  problem  is  a  complex  one,  involv- 
ing, as  it  does,  the  whole  question  of  cell-polarity ;  and  I  know  of 
no  more  promising  field  of  investigation  than  the  ovarian  history  of 
the  ovum  with  reference  to  this  question.  That  Mark's  view  is  cor- 
rect in  principle  is  indicated  by  a  great  array  of  general  evidence 
considered  in  the  following  chapter,  where  its  bearing  on  the  general 
theory  of  development  is  more  fully  dealt  with. 

1  '81,  p.  515.  2  Hatschek  has  suggested  the  same  comparison  {Zoologie,  p.  112). 

*  The  immature  egg  of  Nereis  shows,  however,  a  distinct  polarity  in  the  arrangement   of 
the  fat-drops,  which  form  a  ring  in  the  equatorial  region. 


THE   ENERGY   OF  DIVISION  289 

C.     The  Energy  of  Division 

The  causes  by  which  cell-division  is  incited  and  by  which  its  cessa- 
tion is  determined  are  as  yet  scarcely  comprehended,  and  the  ques- 
tions that  they  suggest  merge  into  the  larger  problem  of  the  general 
control  of  growth.  All  animals  and  plants  have  a  limit  of  growth, 
which  is,  however,  much  more  definite  in  some  forms  than  in  others, 
and  which  differs  in  different  tissues.  During  the  individual  devel- 
opment the  energy  of  cell-division  is  most  intense  in  the  early  stages 
(cleavage)  and  diminishes  more  and  more  as  the  limit  of  growth  is 
approached.  When  the  limit  is  attained  a  more  or  less  definite 
equilibrium  is  established,  some  of  the  cells  ceasing  to  divide  and 
perhaps  losing  this  power  altogether  (nerve-cells),  others  dividing 
only  under  special  conditions  (connective  tissue-cells,  gland-cells, 
muscle-cells),  while  others  continue  to  divide  throughout  life,  and  thus 
replace  the  worn-out  cells  of  the  same  tissue  (Malpighian  layer  of 
the  epidermis,  etc.).  The  limit  of  size  at  which  this  state  of  equi- 
librium is  attained  is  an  hereditary  character,  which  in  many  cases 
shows  an  obvious  relation  to  the  environment,  and  has  therefore  prob- 
ably been  determined  and  is  maintained  by  natural  selection.  From 
the  cytological  point  of  view  the  limit  of  body-size  appears  to  be  cor- 
related with  the  total  mnnbcr  of  cells  formed  rather  than  with  their 
individual  size.  This  relation  has  been  carefully  studied  by  Conklin 
('96)  in  the  case  of  the  gasteropod  Crepidnla,  an  animal  which  varies 
greatly  in  size  in  the  mature  condition,  the  dwarfs  having  in  some 
cases  not  more  than  2^^  the  volume  of  the  giants.  The  eggs  are, 
however,  of  the  same  size  in  all,  and  their  number  is  proportional  to 
the  size  of  the  adult.  The  same  is  true  of  the  tissue-cells.  Measure- 
ments of  cells  from  the  epidermis,  the  kidney,  the  liver,  the  alimen- 
tary epithelium,  and  other  tissues  show  that  they  are  on  the  whole  as 
large  in  the  dwarfs  as  in  the  giants.  The  body-size  therefore  depends 
on  the  total  number  of  cells  rather  than  on  their  size  individually 
considered,  and  the  same  appears  to  be  the  case  in  plants.^ 

Morgan  has  examined  the  same  question  experimentally  through 
a  comparison  of  normal  larvae  of  echinoderms  and  Ampliioxus  with 
dwarf  larvae  of  the  same  species  developed  from  egg-fragments  ('95,  i ; 
'96).  Broadly  speaking,  his  results  agree  with  Conklin's,  though  they 
show  that  the  relation  is  by  no  means  simple  or  constant.  If  unseg- 
mented  eggs  of  sea-urchins  {SpJicerccJiinns)  be  shaken  to  pieces, 
fragments  of  all  sizes  are  obtained  which  may  segment  and  pro- 
duce blastulas  and  gastrulas  ranging  down  to  -^^  the  volume  of 
the  normal  size.  Dwarfs  are  also  obtained  from  isolated  blasto- 
meres   of    two-,  four-,  or  eight-cell  stages.     In  both  cases  the  num- 

1  See  Amelung  ('93)  and  Strasburger  ('93). 


290  CELL-DIVISION  AND  DEVELOPMENT 

ber  of  cells  in  the  blastula  just  before  invagination  is  approximately 
proportional  to  the  size  of  the  blastula,  though  the  smaller  frag- 
ments show  a  tendency  to  produce  a  somewhat  larger  number,  and 
their  cells  are,  therefore,  somewhat  smaller  than  in  the  larger  forms. 
The  same  is  true  in  Amphioxus.  Morgan,  therefore,  draws  the  very 
interesting  conclusion  that  **  the  ultimate  size  of  the  cells  produced 
by  repeated  division  determines  when  the  division  shall  come  to 
an  end  for  a  certain  stage  of  the  ontogeny."  ^  This  conclusion 
is,  however,  subject  to  exception ;  for  Morgan  finds  that  the  dwarf 
larvae  show  a  tendency  to  use  the  same  number  of  cells  in  the 
formation  of  certain  organs  as  the  full-sized  individuals.  Thus  the 
dwarf  blastulas  tend  to  invaginate  the  same  number  of  cells  to 
form  the  archenteron  as  in  the  normal  forms ;  and  in  Amphioxus 
the  notochord  of  the  dwarfs  consists  in  cross-section  of  three  cells, 
as  in  normal  individuals,  irrespective  of  the  total  number  of  cells.  It 
is  clear,  therefore,  that  there  is  another  factor,  besides  the  size  of  the 
cells,  to  be  taken  into  account,  and  the  whole  subject  awaits  further 
investigation. 

The  gradual  diminution  of  the  energy  of  division  during  develop- 
ment by  no  means  proceeds  at  a  uniform  pace  in  all  of  the  cells,  and, 
during  the  cleavage,  the  individual  blastomeres  are  often  found  to 
exhibit  entirely  different  rhythms  of  division,  periods  of  active  division 
being  succeeded  by  long  pauses,  and  sometimes  by  an  entire  cessa- 
tion of  division  even  at  a  very  early  period.  In  the  echinoderms, 
for  example,  it  is  well  established  that  division  suddenly  pauses,  or 
changes  its  rhythm,  just  before  the  gastrulation  (in  Synapta  at  the 
512-cell  stage,  according  to  Selenka),  and  the  same  is  said  to  be 
the  case  in  Amphioxus  (Hatschek,  Lwoff).  In  Nei^eis,  one  of  the 
blastomeres  on  each  side  of  the  body  in  the  forty-two-cell  stage 
suddenly  ceases  to  divide,  migrates  into  the  interior  of  the  body, 
and  is  converted  into  a  unicellular  glandular  organ. ^  In  the  same 
animal,  the  four  lower  cells  (macromeres)  of  the  eight-cell  stage 
divide  in  nearly  regular  succession  up  to  the  thirty-eight-cell  stage, 
when  a  long  pause  takes  place,  and  when  the  divisions  are  re- 
sumed they  are  of  a  character  totally  different  from  those  of  the 
earlier  period.  The  cells  of  the  ciliated  belt  or  prototroch  in  this  and 
other  annelids  likewise  cease  to  divide  at  a  certain  period,  their  numx- 
ber  remaining  fixed  thereafter.^  Again,  the  number  of  cells  produced 
for  the  foundation  of  particular  structures  is  often  definitely  fixed, 
even  when  their  number  is  afterwards   increased  by  division.     In 

i'95,  p.  119. 

2  This  organ,  doubtfully  identified  by  me  as  the  head-kidney,  is  probably  a  mucus-gland 
(Mead). 

3  Cf.  Fig.  1 22. 


THE   ENERGY   OF  DIVISION  29 1 

annelids  and  gasteropods,  for  example,  the  entire  ectoblast  arises 
from  twelve  micromeres  segmented  off  in  three  successive  quartets 
of  micromeres  from  the  blastomeres  of  the  four-cell  stage.  In 
EchinuSy  according  to  Morgan,  the  number  of  cells  used  in  the  forma- 
tion of  the  ^rchenteron  is  approximately  one  hundred;  in  SphcBrechmiis 
the  number  is  approximately  fifty. 

Perhaps  the  most  interesting  numerical  relation  of  this  kind  are 
those  recently  discovered  in  the  division  of  teloblasts,  where  the  num- 
ber of  divisions  is  directly  correlated  with  the  number  of  segments  or 
somites.  It  is  well  known  that  this  is  the  case  in  certain  plants 
{Cliaracece),  where  the  alternating  nodes  and  internodes  of  the  stem 
are  derived  from  corresponding  single  cells  successively  segmented 
off  from  the  apical  cell.  Vejdovsky's  observations  on  the  annehd 
Dendrobcena  give  strong  ground  to  believe  that  the  number  of  meta- 
merically  repeated  parts  of  this  animal,  and  probably  of  other  anne- 
lids, corresponds  in  like  manner  with  that  of  the  number  of  cells 
segmented  off  from  the  teloblasts.  The  most  remarkable  and 
accurately  determined  case  of  this  kind  is  that  of  the  isopod  Crustacea^ 
where  the  number  of  somites  is  limited  and  perfectly  constant.  In  the 
embryos  of  these  animals  there  are  two  groups  of  teloblasts  near  the 
hinder  end  of  the  embryo,  viz.  an  inner  group  of  mesoblasts,  from  which 
arise  the  mesoblast-bands,  and  an  outer  group  of  ectoblasts,  from 
which  arise  the  neural  plates  and  the  ventral  ectoblast.  McMurrich 
('95)  has  recently  demonstrated  that  the  mesoblasts  always  divide 
exactly  sixteen  times,  the  ectoblasts  thirty-two  (or  thirty-three)  times, 
before  relinquishing  their  teloblastic  mode  of  division  and  breaking 
up  into  smaller  cells.  Now  the  sixteen  groups  of  cells  thus  formed 
give  rise  to  the  sixteen  respective  somites  of  the  post-naupliar  region 
of  the  embryo  {i.e.  from  the  second  maxilla  backward).  In  other 
words,  each  single  division  of  the  mesoblasts  and  each  double  division 
of  the  ectoblasts  splits  off  the  material  for  a  single  somite !  The 
number  of  these  divisions,  and  hence  of  the  corresponding  somites, 
is  a  fixed  inheritance  of  the  species. 

The  causes  that  determine  the  rhythm  of  division,  and  thus  finally 
establish  the  adult  equilibrium,  are  but  vaguely  comprehended.  The 
ultimate  causes  must  of  course  lie  in  the  inherited  constitution  of  the 
organism,  and  are  referable  in  the  last  analysis  to  the  structure  of 
the  germ-cells.  Every  division  must,  however,  be  the  response  of  the 
cell  to  a  particular  set  of  conditions  or  stimuli ;  and  it  is  through  the 
investigation  of  these  stimuli  that  we  may  hope  to  penetrate  further 
into  the  nature  of  development.  It  must  be  confessed  that  the 
specific  causes  that  incite  or  inhibit  cell-division  are  scarcely  known. 
The  egg-cell  is  in  most  cases  stimulated  to  divide  by  the  entrance  of 
the  spermatozoon,  but  in  parthenogenesis  exactly  the  same  result  is 


292  CELL-DIVISION  AND  DEVELOPMENT 

produced  by  a  different  cause.  In  the  adult,  cells  may  be  stimulated 
to  divide  by  the  utmost  variety  of  agencies  —  by  chemical  stimulus, 
as  in  the  formation  of  galls,  or  in  hyperplasia  induced  by  the  in- 
jection of  foreign  substances  into  the  blood;  by  mechanical  press- 
ure, as  in  the  formation  of  calluses;  by  injury,  as  in  the  healing  of 
wounds  and  in  the  regeneration  of  lost  parts ;  and  by  a  multitude  of 
more  complex  physiological  and  pathological  conditions,  —  by  any 
agency,  in  short,  that  disturbs  the  normal  equilibrium  of  the  body. 
In  all  these  cases,  however,  it  is  difficult  to  determine  the  immediate 
stimulus  to  division  ;  for  a  long  chain  of  causes  and  effects  may 
intervene  between  the  primary  disturbance  and  the  ultimate  reaction 
of  the  dividing  cells.  Thus  there  is  reason  to  believe  that  the  for- 
mation of  a  callus  is  not  directly  caused  by  pressure  or  friction,  but 
through  the  determination  of  an  increased  blood-supply  to  the  part 
affected  and  a  heightened  nutrition  of  the  cells.  Cell-division  is  here 
probably  incited  by  local  chemical  changes;  and  the  opinion  is  gaining 
ground  that  the  immediate  causes  of  division,  whatever  their  ante- 
cedents, are  to  be  sought  in  this  direction.  The  most  promising  field 
for  their  investigation  seems  to  lie  in  the  direction  of  cellular  pathology 
through  the  study  of  tumours  and  other  abnormal  growths.  The  work 
of  Ziegler  and  Obolonsky  indicates  that  the  cells  of  the  liver  and 
kidney  may  be  directly  incited  to  divide  through  the  action  of  arsenic 
and  phosphorus;  and  several  others  have  reached  analogous  results 
in  the  case  of  other  tissues  and  other  poisons.  The  formation  of 
galls  seems  to  leave  no  doubt  that  extremely  complex  and  charac- 
teristic abnormal  growths  may  result  from  specific  chemical  stimuli, 
and  some  pathologists  have  held  a  similar  view  in  regard  to  the  origin 
of  abnormal  growths  in  the  animal  body. 

Suggestive  as  these  results  are,  they  scarcely  touch  the  ultimate 
problem.  The  unknown  factor  is  that  which  determines  and  main- 
tains the  normal  equilibrium.  A  very  interesting  suggestion  is  the 
resistance  theory  of  Thiersch  and  Boll,  according  to  which  each  tissue 
continues  to  grow  up  to  the  limit  afforded  by  the  resistance  of  neigh- 
bouring tissues  or  organs.  The  removal  or  lessening  of  this  resistance 
through  injury  or  disease  causes  a  resumption  of  growth  and  division, 
leading  either  to  the  regeneration  of  the  lost  parts  or  to  the  forma- 
tion of  abnormal  growths.  Thus  the  removal  of  a  salamander's  limb 
would  seem  to  remove  a  barrier  to  the  proliferation  and  growth 
of  the  remaining  cells.  These  processes  are  therefore  resumed, 
and  continue  until  the  normal  barrier  is  re-established  by  the  re- 
generation. To  speak  of  such  a  "barrier"  or  "resistance"  is,  how- 
ever, to  use  a  highly  figurative  phrase  which  is  not  to  be  construed 
in  a  rude  mechanical  seUvSe.  There  is  no  doubt  that  hypertrophy, 
atrophy,   or   displacement    of    particular    parts   often   leads   to   com- 


CELL-DIVISION  AND    GROWTH  293 

pensatory  changes  in  the  neighbouring  parts ;  but  it  is  equally  certain 
that  such  changes  are  not  a  direct  mechanical  effect  of  the  disturbance, 
but  a  highly  complex  physiological  response  to  it.  How  complex  the 
problem  is^  is  shown  by  the  fact  that  even  closely  related  animals 
may  differ  widely  in  this  respect.  Thus  Fraisse  has  shown  that  the 
salamander  may  completely  regenerate  an  amputated  limb,  while  the 
frog  only  heals  the  wound  without  further  regeneration.^  Again,  in 
the  case  of  coelenterates,  Loeb  and  Bickford  have  shown  that  the 
tubularian  hydroids  are  able  to  regenerate  the  tentacles  at  both  ends 
of  a  segment  of  the  stem,  while  the  polyp  Cerianthus  can  regenerate 
them  only  at  the  distal  end  of  a  section  (Fig.  142).  In  the  latter  case, 
therefore,  the  body  possesses  an  inherent  polarity  which  cannot  be 
overturned  by  external  conditions. 


D.     Cell-Division  and   Growth 

The  relation  between  cell-division  and  growth  has  already  been 
touched  upon  at  pp.  41  and  265.  The  direction  of  the  division- 
planes  in  the  individual  cells  evidently  stands  in  some  causal  rela- 
tion with  the  axes  of  growth  in  the  body,  as  is  especially  clear  in  the 
case  of  rapidly  elongating  structures  (apical  buds,  teloblasts,  and 
the  like),  where  the  division-planes  are  predominantly  transverse  to 
the  axis  of  elongation.  Which  of  these  is  the  primary  factor,  the 
direction  of  general  growth  or  the  direction  of  the  division-planes  t 
This  question  is  a  difficult  one  to  answer,  for  the  two  phenomena 
are  often  too  closely  related  to  be  disentangled.  As  far  as  the 
plants  are  concerned,  however,  it  has  been  conclusively  shown  by 
Hofmeister,  De  Bary,  and  Sachs  that  tJie  growth  of  the  mass  is  the 
primary  factor ;  for  the  characteristic  mode  of  growth  is  often  shown 
by  the  growing  mass  before  it  splits  up  into  cells,  and  the  form  of 
cell-division  adapts  itself  to  that  of  the  mass  :  *'  Die  Pflanze  bildet 
Zellen,  nicht  die  Zelle  bildet  Pflanzen  "  (De  Bary). 

The  opinion  has  of  late  rapidly  gained  ground  that  the  same  is 
true  in  principle  of  animal  growth,  and  this  view  has  been  urged 
by  many  writers,  among  whom  may  be  mentioned  Rauber,  Hertwig, 
and  especially  Whitman,  whose  fine  essay  on  the  hiadequacy  of  the 
Ccll-tJicory  of  Development  ('93)  marks  a  distinct  advance  in  our 
point  of  view.  It  is  certain  that  in  the  earlier  stages  of  develop- 
ment, and  in  a  less  degree  in  later  stages  as  well,  the  character  of 
growth  and  division  in  the  individual  cell  is  but  a  local  manifesta- 
tion of  a  formative  power  pervading  the  organism  as  a  whole ;  and 

1  In  salamanders  regeneration  only  takes  place  when  the  bone  is  cut  across,  and  does  not 
occur  if  the  limb  be  exarticulated  and  removed  at  the  joint. 


294  CELL-DIVISION  AND  DEVELOPMENT 

the  general  truth  of  this  view  has  been  in  certain  cases  conclusively 
demonstrated  by  experiment.^  It  has,  however,  become  clear  that 
this  conclusion  can  be  accepted  only  with  certain  reservations ;  for 
as  development  proceeds,  the  cells  may  acquire  so  high  a  degree 
of  independence  that  profound  modifications  may  occur  in  special 
regions  through  injury  or  disease,  without  affecting  the  general  equi- 
librium of  the  body.  The  most  striking  proof  of  this  lies  in  the 
fact  that  grafts  or  transplanted  structures  may  perfectly  retain 
their  specific  character,  though  transferred  to  a  different  region  of 
the  body,  or  even  to  another  species.  Nevertheless  the  facts  of 
regeneration  prove  that  even  in  the  adult  the  formative  processes  in 
special  parts  are  in  many  cases  definitely  correlated  with  the  organ- 
ization of  the  entire  mass ;  and  in  the  following  chapter  we  shall  see 
reason  to  conclude  that  such  a  correlation  is  a  survival,  in  the  adult, 
of  a  condition  characteristic  of  the  embryonic  stages,  and  that  the 
independence  of  special  parts  in  the  adult  is  a  secondary  result  of 
development. 

LITERATURE.     VIII 

Berthold,  G.  —  Studien  liber  Protoplasma-mechanik.     Leipzig,  1886. 

Boll,  Fr.  —  Das  Princip  des  Wachsthums.     Berlitij  1876. 

Bourne,  G.  C.  —  A  Criticism  of  the  Cell-theory ;  being  an  answer  to  Mr.  Sedgwick's 

article  on   the  Inadequacy  of  the   Cellular  Theory  of  Development :    Quart. 

Joiirn.  M.  S.,  XXXVIII.  i,  1895. 
Errara.  —  Zellformen   und    Seifenblasm :    Tagebl.    der   60    Versaj7iinliuig  deiitscher 

Naturforscher  und  Aerzte  zti  Wiesbade7i.     1887. 
Hertwig,  0.  —  Das   Problem   der   Befruchtung  und   der    Isotropie   des    Eies,  eine 

Theorie  der  Vererbung.     Jena,  1884. 
Hofmeister.  —  Die  Lehre  von  der  Pflanzenzelle.     Leipzig,  1867. 
McMurrich,  J.  P.  —  Embryology  of  the  Isopod  Crustacea:  Journ.  Morph.,X\.  i. 

1895. 
Mark,  E.  L.  —  Limax.     (See  List  IV.) 
Rauber,  A.  —  Neue  Grundlegungen  zur  Kenntniss  der  Zelle:  Morph.  Jahrb.,V\\\. 

1883. 
Sachs,  J.  —  Pflanzenphysiologie.     (See  List  VII.) 
Sedgwick,  H. — On  the  Inadequacy  of  the  Cellular  Theory  of  Development,  etc.: 

Quart .  Journ .  Mic.  Sci.,XXXV\\.  \.     1 894 . 
Strasburger,  E.  —  Ueber  die  Wirkungssphare  der  Kerne  und  die  Zellgrosse :  Histo- 

logische  Beit  rage,  V.     1893. 
Watase,  S.  —  Studies  on  Cephalopods  ;  I.,  Cleavage  of  the  Ovum  :  Journ.  Morph., 

IV.  3.      1891. 
Whitman,  C.  0.  —  The  Inadequacy  of  the  Cell-theory  of  Development :  Wood's  Holl 

Biol.  Lectures.     1893. 
Wilson,  Edm.  B.  —  The  Cell-lineage  oi  Nereis :  Jourti.  Morph.,  VI.  3.     1892.  • 
Id.  —  Amphioxus  and  the  Mosaic  Theory  of  Development:    Journ.  Morph.,  VIII. 

3.     1893. 

1  Cf.  p.  312. 


CHAPTER    IX 


THEORIES   OF    INHERITANCE   AND    DEVELOPMENT 

"  It  is  certain  that  the  germ  is  not  merely  a  body  in  which  life  is  dormant  or  potential, 
but  that  it  is  itself  simply  a  detached  portion  of  the  substance  of  a  pre-existing  living  body." 

HUXLEY.I 

**  Inheritance  must  be  looked  at  as  merely  a  form  of  growth."  Darwin.2 

"  Ich  mochte  daher  wohl  den  Versuch  wagen,  durch  eine  Darstellung  des  Beobachteten 
Sie  zu  einer  tiefern  Einsicht  in  die  Zeugungs-  und  Entwickelungsgeschichte  der  organischen 
Korper  zu  fiihren  und  zu  zeigen,  wie  dieselben  weder  vorgebildet  sind,  noch  auch,  wie  man 
sich  gewohnlich  denkt,  aus  ungeformter  Masse  in  einem  bestimmten  Momente  plotzlich 
ausschiessen."  VoN  Baer.^ 

Every  discussion  of  inheritance  and  development  must  take  as  its 
point  of  departure  the  fact  that  the  germ  is  a  single  cell  similar  in 
its  essential  nature  to  any  one  of  the  tissue-cells  of  which  the  body- 
is  composed.  That  a  cell  can  carry  with  it  the  sum  total  of  the 
heritage  of  the  species,  that  it  can  in  the  course  of  a  few  days  or 
weeks  give  rise  to  a  mollusk  or  a  man,  is  the  greatest  marvel  of 
biological  science.  In  attempting  to  analyze  the  problems  that  it 
involves,  we  must  from  the  outset  hold  fast  to  the  fact,  on  which 
Huxley  insisted,  that  the  wonderful  formative  energy  of  the  germ  is 
not  impressed  upon  it  from  without,  but  is  inherent  in  the  egg  as 
a  heritage  from  the  parental  life  of  which  it  was  originally  a  part. 
The  development  of  the  embryo  is  nothing  new.  It  involves  no 
breach  of  continuity,  and  is  but  a  continuation  of  the  vital  pro- 
cesses going  on  in  the  parental  body.  What  gives  development  its 
marvellous  character  is  the  rapidity  with  which  it  proceeds  and  the 
diversity  of  the  results  attained  in  a  span  so  brief. 

But  when  we  have  grasped  this  cardinal  fact  we  have  but  focussed 
our  instruments  for  a  study  of  the  real  problem.  Hozv  do  the  adult 
characteristics  lie  latent  in  the  germ-cell ;  and  how  do  they  become 
patent  as  development  proceeds  .''  This  is  the  final  question  that 
looms   in  the  background   of   every   investigation   of    the   cell.       In 

^  Evolution,  Science  and  Culture,  p.  291. 

2  Variation  of  Animals  and  Plants,  II.  p.  398. 

3  Enttuick.  der  Thiere,  II.,  1837,  p.  8. 

295 


296  THEORIES   OF  INHERITANCE   AND  DEVELOPMENT 

approaching  it  we  may  well  make  a  frank  confession  of  ignorance ; 
for  in  spite  of  all  that  the  microscope  has  revealed,  we.  have  not 
yet  penetrated  the  mystery,  and  inheritance  and  development  still 
remain  in  their  fundamental  aspects  as  great  a  riddle  as  they  were 
to  the  Greeks.  What  we  have  gained  is  a  tolerably  precise  acquaint- 
ance with  the  external  aspects  of  development.  The  gross  errors  of 
the  early  preformationists  have  been  dispelled. ^  We  know  that  the 
germ-cell  contains  no  predelineated  embryo ;  that  development  is 
manifested,  on  the  one  hand,  by  a  continued  process  of  cell-division, 
on  the  other  hand,  by  a  process  of  differentiation,  through  which 
the  cells  gradually  assume  diverse  forms  and  functions,  and  so 
accomplish  a  physiological  division  of  labour.  But  we  have  not  yet 
fathomed  the  inmost  structure  of  the  germ-cell,  and  the  means  by 
which  the  latent  adult  characters  that  it  involves  are  made  actual 
as  development  proceeds.  And  it  should  be  clearly  understood  that 
when  we  attempt  to  approach  these  deeper  problems  we  are  com- 
pelled to  advance  beyond  the  solid  ground  of  fact  into  a  region  of 
more  or  less  doubtful  and  shifting  hypothesis,  where  the  point  of 
view  continually  changes  as  we  proceed.  It  would,  however,  be  an 
error  to  conclude  that  modern  hypotheses  of  inheritance  and  develop- 
ment are  baseless  speculations  that  attempt  a  merely  formal  solution 
of  the  problem,  like  those  of  the  seventeenth  and  eighteenth  cen- 
turies. They  are  a  product  of  the  inductive  method,  a  direct  out- 
come of  accurately  determined  fact,  and  they  lend  to  the  study  of 
embryology  a  point  and  precision  that  it  would  largely  lack  if  limited 
to  a  strictly  objective  description  of  phenomena. 

All  discussions  of  development  are  now  revolving  about  two  cen- 
tral hypotheses,  a  preliminary  examination  of  which  will  serve  as 
an  introduction  to  the  general  subject.  These  are,  first,  the  theory 
of  Germinal  Localization'^  of  Wilhelm  His  ('74),  and  second,  the 
Idioplasm  Hypothesis  of  Nageli  ('84).  The  relation  between  these 
two  conceptions,  close  as  it  is,  is  not  at  first  sight  very  apparent ;  and 
for  the  purpose  of  a  preliminary  sketch  they  may  best  be  considered 
separately. 


A.     The  Theory  of  Germinal  Localization 

Although  the  naive  early  theory  of  preformation  and  evolution 
was  long  since  abandoned,  yet  we  find  an  after-image  of  it  in  the 
theory  of  germinal  localization  which   in   one  form  or  another  has 

1  Cf.  Introduction,  p.  6. 

2  I  venture  to  suggest  this  term  as  an  English  equivalent  for  the  awkward  expression 
"  Organbildende  Keimbezirke  "  of  His. 


THE    THEORY   OF  GERMINAL   LOCALIZATION  297 

been  advocated  by  some  of  the  foremost  students  of  development. 
It  is  maintained  that,  although  the  embryo  is  not  ^x^-foimed  in 
the  germ,  it  must  nevertheless  be  -prQ-deteruiined  in  the  sense  that  the 
Qgg  contains  definite  areas  or  definite  substances  predestined  for  the 
formation  of  corresponding  parts  of  the  embryonic  body.  The  first 
definite  statement  of  this  conception  is  found  in  the  interesting  and 
suggestive  work  of  Wilhelm  His  ('74)  entitled  Unsere  Korperform. 
Considering  the  development  of  the  chick,  he  says  :  *'  It  is  clear,  on 
the  one  hand,  that  every  point  in  the  embryonic  region  of  the  blasto- 
derm must  represent  a  later  organ  or  part  of  an  organ,  and  on  the 
other  hand,  that  every  organ  developed  from  the  blastoderm  has 
its  preformed  germ  ("  vorgebildete  Anlage  ")  in  a  definitely  located 
region  of  the  flat  germ-disc.  .  .  .  The  material  of  the  germ  is 
already  present  in  the  flat  germ-disc,  but  is  not  yet  morphologically 
marked  off  and  hence  not  directly  recognizable.  But  by  following 
the  development  backwards  we  may  determine  the  location  of  every 
such  germ,  even  at  a  period  when  the  morphological  differentiation 
is  incomplete  or  before  it  occurs ;  logically,  indeed,  we  must  extend 
this  process  back  to  the  fertilized  or  even  the  unfertilized  ^gg. 
According  to  this  principle,  the  germ-disc  contains  the  organ-germs 
spread  out  in  a  flat  plate,  and,  conversely,  every  point  of  the  germ- 
disc  reappears  in  a  later  organ  ;  I  call  this  the  principle  of  orgafi- 
formiiig  germ-regions''  ^  His  thus  conceived  the  embryo,  not  as 
"prQ-fornied,  but  as  having  all  of  its  parts  ^x^-localized  in  the  egg- 
protoplasm  (cytoplasm). 

A  great  impulse  to  this  conception  was  given  during  the  following 
decade  by  discoveries  relating,  on  the  one  hand,  to  protoplasmic 
structure,  on  the  other  hand,  to  the  promorphological  relations  of  the 
ovum.  Ray  Lankester  writes,  in  1877:  ^'Though  the  substance  of  a 
celP  may  appear  homogeneous  under  the  most  powerful  microscope, 
it  is  quite  possible,  indeed  certain,  that  it  may  contain,  already  formed 
and  individnalized,  various  kinds  of  physiological  molecules.  The 
visible  process  of  segregation  is  only  the  sequel  of  a  differentiation 
already  established,  and  not  visible."^  The  egg-cytoplasm  has  a  defi- 
nite molecular  organization  directly  handed  down  from  the  parent ; 
cleavage  sunders  the  various  '*  physiological  molecules  "  and  iso- 
lates them  in  particular  cells.  Whitman  expresses  a  similar  thought 
in  the  following  year :  "  While  we  cannot  say  that  the  embryo  is 
predelineated,  we  can  say  that  it  is  predetermined.  The  '  Histo- 
genetic  sundering '  of  embryonic  elements  begins  with  the  cleavage, 

1  I.e.,  p.  19. 

-  It  is  clear  from  the  context  that  by  "  substance"  Lankester  had  in  mind  the  cytoplasm, 
though  this  is  not  specifically  stated. 
3  '77,  p.  14. 


298  THEORIES   OF  INHERITANCE   AND  DEVELOPMENT 

and  every  step  in  the  process  bears  a  definite  and  invariable  relation 
to  antecedent  and  subsequent  steps.  ...  It  is,  therefore,  not  sur- 
prising to  find  certain  important  histological  differentiations  and 
fundamental  structural  relations  anticipated  in  the  early  phases  of 
cleavage,  and  foreshadowed  even  before  cleavage  begins."  ^  It  was, 
however,  Flemming  who  gave  the  first  specific  statement  of  the 
matter  from  the  cytological  point  of  view  :  "  But  if  the  substance  of 
the  egg-cell  has  a  definite  structure  (Bau),  and  if  this  structure  and 
the  nature  of  the  network  varies  in  different  regions  of  the  cell- 
body,  we  may  seek  in  it  a  basis  for  the  predetermination  of  develop- 
ment wherein  one  ^gg  differs  from  another,  and  it  will  be  possible  to 
look  for  it  with  the  microscope.  How  far  this  search  can  be  carried 
no  one  can  say,  but  its  ultimate,  aim  is  nothing  less  than  a  true 
morphology  of  inheritance?  In  the  following  year  Van  Beneden 
pointed  out  how  nearly  this  conception  approaches  to  a  theory  of 
preformation :  "  If  this  were  the  case  {i.e.  if  the  egg-axis  coincided 
with  the  principal  axis  of  the  adult  body),  the  old  theory  of  evolution 
would  not  be  as  baseless  as  we  think  to-day.  The  fact  that  in  the 
ascidians,  and  probably  in  other  bilateral  animals,  the  median  plane 
of  the  body  of  the  future  animal  is  marked  out  from  the  beginning  of 
cleavage,  fully  justifies  the  hypothesis  that  the  materials  destined  to 
form  the  right  side  of  the  body  are  situated  in  one  of  the  lateral 
hemispheres  of  the  ^gg,  while  the  left  hemisphere  gives  rise  to  all  of 
the  organs  of  the  left  half."  ^ 

The  hypothesis  thus  suggested  seemed,  for  a  time,  to  be  placed  on 
a  secure  basis  of  fact  through  a  remarkable  experiment  subsequently 
performed  by  Roux  i^'^^)  on  the  frog's  ^gg.  On  killing  one  of  the 
blastomeres  of  the  two-cell  stage  by  means  of  a  heated  needle  the 
uninjured  half  developed  in  some  cases  into  a  perfectly  formed  half- 
lai-va  (Fig.  131),  accurately  representing  the  right  or  left  half  of  the 
body,  containing  one  medullary  fold,  one  auditory  pit,  etc.*  Analo- 
gous, though  less  complete,  results  were  obtained  by  operating  with 
the  four-cell  stage.  Roux  was  thus  led  to  the  declaration  (made 
with  certain  subsequent  reservations)  that  "the  development  of  the 
frog-gastrula  and  of  the  embryo  formed  from  it  is  from  the  second 
cleavage  onward  a  mosaic-work  consisting  of  at   least  four  vertical 

1  '78,  p.  49. 

2  Zellsubstanz,  '82,  p.  70;   the  italics  are  in  the  original. 

'  '83.  p.  571- 

*  The  accuracy  of  this  result  was  disputed  by  Oscar  Hertwig  ('93,  i),  who  found  that  the 
uninjured  blastomere  gave  rise  to  a  defective  larva,  in  which  certain  parts  were  missing,  but 
not  to  a  true  half-body.  Later  observers,  especially  Schultze,  Endres,  and  Morgan,  have, 
however,  shown  that  both  Hertwig  and  Roux  were  right,  proving  that  the  uninjured  blasto- 
mere may  give  rise  to  a  perfect  half-larva,  to  a  larva  with  irregular  defects,  or  to  a  whole 
larva  of  half-size,  according  to  circumstances  (p.  319). 


THE    THEORY   OF  GERMINAL   LOCALIZATION 


299 


independently  developing  pieces."  ^  This  conclusion  seemed  to  form 
a  very  strong  support  to  His's  theory  of  germinal  localization, 
though,  as  will  appear  beyond,  Roux  transferred  this  theory  to  the 
nucleus,  and  thus  developed  it  in  a  very  different  direction  from 
Lankester  or  Van  Beneden. 


Fig.  131.  —  Half-embryos  of  the  frog  (in  transverse  section)  arising  from  a  blastomere  of  the 
2-cell  stage  after  killing  the  other  blastomere.     [Roux.] 

A.  Half-blastula  (dead  blastomere  on  the  left).  D.  Later  stage.  C.  Half-tadpole  with  one 
medullary  fold  and  one  mesoblast  plate;  regeneration  of  the  missing  (right)  half  in  process. 

ar.,  archenteric  cavity :  c.c,  cleavage-cavity;  ch,  notochord ;  w,/!,  medullary  fold ;  /«j.,  meso- 
blast-plate. 

In  an  able  series  of  later  works  Whitman  has  followed  out  the  sug- 
gestion made  in  his  paper  of  1878,  already  cited,  pointing  out  how 
essential  a  part  is  played  in  development  by  the  cytoplasm  and  insist- 
ing that  cytoplasmic  pre-organization  must  be  regarded  as  a  leading 
factor  in  the  ontogeny.     Whitman's  interesting  and  suggestive  views 

1  Lc,  p.  30. 


300  THEORIES   OF  INHERITANCE    AND   DEVELOPMENT 

are  expressed  with  great  caution  and  with  a  full  recognition  of  the 
difficulty  and  complexity  of  the  problem.  From  his  latest  essay,  in- 
deed ('94),  it  is  not  easy  to  gather  his  precise  position  regarding  the 
theory  of  cytoplasmic  localization.  Through  all  his  writings,  never- 
theless, runs  the  leading  idea  that  the  germ  is  definitely  organized 
before  development  begins,  and  that  cleavage  only  reveals  an  organ- 
ization that  exists  from  the  beginning.  '*  That  organization  precedes 
cell-formation  and  regulates  it,  rather  than  the  reverse,  is  a  conclu- 
sion that  forces  itself  upon  us  from  many  sides."  ^  "The  organ- 
ism exists  before  cleavage  sets  in,  and  persists  throughout  every 
stage  of  cell-multiplication."  2  In  so  far  as  this  view  involves  the 
assumption  that  the  organization  of  the  egg-cytoplasm  at  the  be- 
ginning of  cleavage  is  a  primordial  character  of  the  ^gg,  Whitman's 
conception  must,  I  think,  be  placed  on  the  side  of  the  localization 
theory ;  but  his  point  of  view  can  only  be  appreciated  through  a 
study  of  his  own  writings. 

All  of  these  views,  excepting  those  of  Roux,  lean  more  or  less 
distinctly  towards  the  conclusion  that  the  cytoplasm  of  the  egg-cell 
is  from  the  first  mapped  out,  as  it  were,  into  regions  which  corre- 
spond with  the  parts  of  the  future  embryonic  body.  The  cleavage 
of  the  ovum  does  not  create  these  regions,  but  only  reveals  them  to 
view  by  marking  off  their  boundaries.  Their  topographical  arrange- 
ment in  the  ^%g  does  not  necessarily  coincide  with  that  of  the  adult 
parts,  but  only  involves  the  latter  as  a  necessary  consequence  —  some- 
what as  a  picture  in  the  kaleidoscope  gives  rise  to  a  succeeding  pic- 
ture composed  of  the  same  parts  in  a  different  arrangement.  The 
germinal  localization  may,  however,  in  a  greater  or  less  degree,  fore- 
shadow the  arrangement  of  adult  parts  —  for  instance,  in  the  Qgg  of 
the  tunicate  or  cephalopod,  where  the  bilateral  symmetry  and  antero- 
posterior differentiation  of  the  adult  is  foreshadowed  not  only  in  the 
cleavage  stages,  but  even  in  the  unsegmented  Q.gg. 

By  another  set  of  writers,  such  as  Roux,  De  Vries,  Hertwig,  and 
Weismann,  germinal  localization  is  primarily  sought  not  in  the  cyto- 
plasm, but  in  the  nucleus ;  but  these  views  can  best  be  considered 
after  a  review  of  the  idioplasm  hypothesis,  to  which  we  now  proceed. 


B.     The  Idioplasm  Theory 

We  owe  to  Nageli  the  first  systematic  attempt  to  discuss  heredity 
regarded  as  inherent  in  a  definite  physical  basis ;  ^  but  it  is  hardly 
necessary  to  point  out  his  great  debt  to  earlier  writers,  foremost 
among    them   Darwin,    Herbert    Spencer,   and    Hackcl.'    It  was  the 

-  I.e.,  p.  112.  3  Theorie  der  Abstammungslehre,  1884. 


THE   IDIOPLASM   THEORY  3OI 

great  merit  of  Nageli's  hypothesis  to  consider  inheritance  as  effected 
by  the  transmission  not  of  a  cell,  considered  as  a  whole,  but  of  a  par- 
ticular substance,  the  idioplasm,  contained  within  a  cell,  and  forming 
the  physical  basis  of  heredity.  The  idioplasm  is  to  be  sharply  dis- 
tinguished from  the  other  constituents  of  the  cell,  which  play  no 
direct  part  in  inheritance  and  form  a  '*  nutritive  plasma "  or 
tropJioplasuL.  Hereditary  traits  are  the  outcome  of  a  definite  molec- 
ular organization  of  the  idioplasm.  The  hen's  ^^g  differs  from  the 
frog's  because  it  contains  a  different  idioplasm.  The  species  is  as 
completely  contained  in  the  one  as  in  the  other,  and  the  hen's  ^^^ 
differs  from  a  frog's  as  widely  as  a  hen  from  a  frog. 

The  idioplasm  was  conceived  as  an  extremely  complex  substance 
consisting  of  elementary  complexes  of  molecules  known  as  micellce. 
These  are  variously  grouped  to  form  units  of  higher  orders,  which, 
as  development  proceeds,  determine  the  development  of  the  adult 
cells,  tissues,  and  organs.  The  specific  peculiarities  of  the  idioplasm 
are  therefore  due  to  the  arrangement  of  the  micellae ;  and  this,  in  its 
turn,  is  owing  to  dynamic  properties  of  the  micellae  themselves.  Dur- 
ing development  the  idioplasm  undergoes  a  progressive  transforma- 
tion of  its  substance,  not  through  any  material  change,  but  through 
dynamic  alterations  of  the  conditions  of  tension  and  movement  of 
the  micellae.  These  changes  in  the  idioplasm  cause  reactions  on  the 
part  of  surrounding  structures  leading  to  definite  chemical  and  plastic 
changes,  i.e.  to  differentiation  and  development. 

Nageli  made  no  attempt  to  locate  the  idioplasm  precisely  or  to 
identify  it  with  any  of  the  known  morphological  constituents  of  the 
cell.  It  was  somewhat  vaguely  conceived  as  a  network  extending 
through  both  nucleus  and  cytoplasm,  and  from  cell  to  cell  through- 
out the  entire  organism.  Almost  immediately  after  the  publication 
of  his  theory,  however,  several  of  the  foremost  leaders  of  biologi- 
cal investigation  were  led  to  locate  the  idioplasm  in  the  nucleus, 
and  succeeding  researches  have  rendered  it  more  and  more  highly 
probable  that  it  is  to  be  identified  with  cJiromatiit.  The  grounds 
for  this  conclusion,  which  have  already  been  stated  in  Chapter 
VII.,  may  be  here  again  briefly  reviewed.  The  beautiful  experi- 
ments of  Nussbaum,  Gruber,  and  Verworn  proved  that  the  regenera- 
tion of  differentiated  cytoplasmic  structures  in  the  Protozoa  can  only 
take  place  when  nuclear  matter  is  present  (cf.  p.  248).  The  study  of 
fertilization  by  Hertwig,  Strasburger,  and  Van  Beneden  proved  that 
in  the  sexual  reproduction  of  both  plants  and  animals  the  nucleus  of 
the  germ  is  equally  derived  from  both  sexes,  while  the  cytoplasm  is 
derived  almost  entirely  from  the  female.  The  two  germ-nuclei,  which 
by  their  union  give  rise  to  that  of  the  germ,  were  shown  by  Van 
Beneden  to  be  of  exactly  the  same  morphological  nature,  since  each 


302  THEORIES    OF  INHERITANCE   AND  DEVELOPMENT 

gives  rise  to  chromosomes  of  the  same  number,  form,  and  size.  Van 
Beneden  and  Boveri  proved  (p.  134)  that  the  paternal  and  maternal 
nuclear  substances  are  equally  distributed  to  each  of  the  first  two 
cells,  and  the  more  recent  work  of  Hacker,  Riickert,  Herla,  and 
Zoja  establishes  a  strong  probability  that  this  equal  distribution  con- 
tinues in  the  later  divisions.  Roux  pointed  out  the  telling  fact  that 
the  entire  complicated  mechanism  of  mitosis  seems  designed  to  effect 
the  most  accurate  division  of  the  entire  nuclear  substance  in  all  of 
its  parts,  while  fission  of  the  cytoplasmic  cell-body  is  in  the  main  a 
mass-division,  and  not  a  meristic  division  of  the  individual  parts. 
Again,  the  complicated  processes  of  maturation  show  the  significant 
fact  that  while  the  greatest  pains  is  taken  to  prepare  the  germ-nuclei 
for  their  coming  union,  by  rendering  them  exactly  equivalent,  the 
cytoplasm  becomes  widely  different  in  the  two  germ-cells  and  is 
devoted  to  entirely  different  functions. 

It  was  in  the  main  these  considerations  that  led  Hertwig,  Stras- 
burger,  Kolliker,  and  Weismann  independently  and  almost  simultane- 
ously to  the  conclusion  that  the  nucleus  contains  the  physical  basis  of 
inheritance,  and  that  chromatin,  its  essential  constituent,  is  the  idio- 
plasm postulated  in  Ndgeli  s  theory.  This  conclusion  is  now. widely 
accepted ;  and  notwithstanding  certain  facts  which  at  first  sight  may 
seem  opposed  to  it,  I  believe  it  rests  upon  a  basis  so  firm  that  it  may 
be  taken  as  one  of  the  elementary  data  of  heredity.  To  accept  it  is, 
however,  to  reject  the  theory  of  germinal  localization  in  so  far  as  it 
assumes  a  pre-localization  of  the  egg-cytoplasm  as  a  fundamental 
character  of  the  Q,gg.  For  if  the  specific  character  of  the  organism  be 
determined  by  an  idioplasm  contained  in  the  chromatin,  then  every 
characteristic  of  the  cytoplasm  must  in  the  long  run  be  determined 
from  the  same  source.  A  striking  illustration  of  this  fact  is  given 
by  the  phenomena  of  colour-inheritance  in  plant-hybrids,  as  De  Vries 
has  pointed  out.  Pigment  is  developed  in  the  embryonic  cytoplasm, 
which  is  derived  from  the  mother-cell ;  yet  in  hybrids  it  may  be 
inherited  from  the  male  through  the  nucleus  of  the  germ-cell.  The 
specific  form  of  cytoplasmic  metabolism  by  which  the  pigment  is 
formed  must  therefore  be  determined  by  the  paternal  chromatin  in 
the  germ-nucleus,  and  not  by  a  pre-determination  of  the  egg-cyto- 
plasm. 

C.     Union  of  the  Two  Theories 

We  have  now  to  consider  the  attempts  that  have  been  made  to 
transfer  the  localization-theory  from  the  cytoplasm  to  the  nucleus, 
and  thus  to  bring  it  into  harmony  with  the  theory  of  nuclear  idio- 
plasm.    These  attempts  are  especially  associated  with  the  names  of 


THE  ROUX-WEISMANN   THEORY   OF  DEVELOPMENT  303 

Roux,  De  Vries,  Weismann,  and  Hertwig ;  but  all  of  them  may  be 
traced  back  to  Darwin's  celebrated  hypothesis  of  pangenesis  as  a 
prototype.  This  hypgthesis  is  so  well  known  as  to  require  but  a 
brief  review.  Its  fundamental  postulate  assumes  that  the  germ-cells 
contain  innumerable  ultra-microscopic  organized  bodies  or  gejnmtiles, 
each  of  which  is  the  germ  of  a  cell  and  determines  the  development 
of  a  similar  cell  during  the  ontogeny.  The  germ-cell  is,  therefore,  in 
Darwin's  words,  a  microcosm  formed  of  a  host  of  inconceivably  mi- 
nute self-propagating  organisms,  every  one  of  which  predetermines 
the  formation  of  one  of  the  adult  cells.  De  Vries  ('89)  brought  this 
conception  into  relation  with  the  theory  of  nuclear  idioplasm  by 
assuming  that  the  gemmules  of  Darwin,  which  he  c^W^dpangens,  are 
contained  in  the  nucleus,  migrating  thence  into  the  cytoplasm  step 
by  step  during  ontogeny,  and  thus  determining  the  successive  stages 
of  development.  The  same  view  was  afterwards  accepted  by  Hert- 
wig and  Weismann.^ 

The  theory  of  germinal  localization  is  thus  transferred  from  the 
cytoplasm  to  the  nucleus.  It  is  not  denied  that  the  egg-cytoplasm 
may  be  more  or  less  distinctly  differentiated  into  regions  that  have  a 
constant  relation  to  the  parts  of  the  embryo.  This  differentiation  is, 
however,  conceived,  not  as  a  primordial  characteristic  of  the  ^gg,  but 
as  one  secondarily  determined  through  the  influence  of  the  nucleus. 
Both  De  Vries  and  Weismann  assume,  in  fact,  that  the  entire  cyto- 
plasm is  a  product  of  the  nucleus,  being  composed  of  pangens  that 
migrate  out  from  the  latter,  and  by  their  active  growth  and  multipli- 
cation build  up  the  cytoplasmic  substance.^ 


D.     The  Roux-Weismann  Theory  of  Development 

We  now  proceed  to  an  examination  of  two  sharply  opposing  hy- 
potheses of  developrnent  based  on  the  theory  of  nuclear  idioplasm. 
One  of  these  originated  with  Roux  ('83)  and  has  been  elaborated 
especially  by  Weismann.  The  other  was  clearly  outHned  by  De  Vries 
('89),  and  has  been  developed  in  various  directions  by  Oscar  Hertwig, 

1  The  neo-pangenesis  of  De  Vries  differs  from  Darwin's  hypothesis  in  one  very  important 
respect.  Darwin  assumed  that  the  gemmules  arose  in  the  body,  being  thrown  off  as  germs 
by  the  individual  tissue-cells,  transported  to  the  germ-cells,  and  there  accumulated  as  in  a 
reservoir;  and  he  thus  endeavoured  to  explain  the  transmission  of  acquired  characters.  De 
Vries,  on  the  other  hand,  denies  such  a  transportal  from  cell  to  cell,  maintaining  that  the 
pangens  arise  or  pre-exist  in  the  germ-cell,  and  those  of  the  tissue-cells  are  derived  from  this 
source  by  cell-division. 

-  This  conception  obviously  harmonizes  with  the  role  of  the  nucleus  in  the  synthetic 
process.  In  accepting  the  view  that  the  nuclear  control  of  the  cell  is  effected  by  an  emana- 
tion of  specific  substances  from  the  nucleus,  we  need  not,  however,  necessarily  adopt  the 
pangen-hypothesis. 


304  THEORIES   OF  INHERITANCE   AND  DEVELOPMENT 

Driesch,  and  other  writers.  In  discussing  them,  it  should  be  borne 
in  mind  that,  although  both  have  been  especially  developed  by  the 
advocates  of  the  pangen-hypothesis,  neither  necessarily  involves  that 
hypothesis  in  its  strict  form,  i.e.  the  postulate  of  discrete  self-propa- 
gating units  in  the  idioplasm.  This  hypothesis  may  therefore  be  laid 
aside  as  an  open  question,  and  will  be  considered  only  in  so  far  as  it 
is  necessary  to  a  presentation  of  the  views  of  individual  writers. 

The  Roux-Weismann  hypothesis  has  already  been  touched  on  at 
p.  183.  Roux  conceived  the  idioplasm  {i.e.  the  chromatin)  not  as  a 
single  chemical  compound  or  a  homogeneous  mass  of  molecules,  but 
as  a  highly  complex  mixture  of  different  substances,  representing 
different  qualities,  and  having  their  seat  in  the  individual  chromatin- 
granules.  In  mitosis  these  become  arranged  in  a  linear  series  to 
form  the  spireme-thread,  and  hence  may  be  precisely  divided  by  the 
splitting  of  the  thread.  Roux  assumes,  as  a  fundamental  postulate, 
that  division  of  the  granules  may  be  either  qiiaittitative  or  qjialitative. 
In  the  first  mode  the  group  of  qualities  represented  in  the  mother- 
granule  is  first  doubled  and  then  split  into  equivalent  daughter-groups, 
the  daughter-cells  therefore  receiving  the  same  qualities  and  remain- 
ing of  the  same  nature.  In  *'  qualitative  division,"  on  the  other  hand, 
the  mother-group  of  qualities  is  split  into  dissimilar  groups,  which, 
passing  into  the  respective  daughter-nuclei,  lead  to  a  corresponding 
differentiation  in  the  daitgJiter-cells.  By  qualitative  divisions,  occur- 
ring in  a  fixed  and  predetermined  order,  the  idioplasm  is  thus  split 
up  during  ontogeny  into  its  constituent  qualities,  which  are,  as  it  were, 
sifted  apart  and  distributed  to  the  various  nuclei  of  the  embryo. 
Every  cell-nucleus,  therefore,  receives  a  specific  form  of  chivniatin  which 
•determines  the  nature  of  the  cell  at  a  given  period  and  its  later  his- 
tory. Every  cell  is  thus  endowed  with  a  power  of  self  determination, 
which  lies  in  the  specific  structure  of  its  nucleus,  and  its  course  of 
•development  is  only  in  a  minor  degree  capable  of  modification  through 
the  relation  of  the  cell  to  its  fellows   ("  correlative  differentiation  "). 

Roux's  hypothesis,  be  it  observed,  does  not  commit  him  to  the 
theory  of  pangenesis.  It  was  reserved  for  Weismann  to  develop  the 
hypothesis  of  qualitative  division  in  terms  of  the  pangen-hypothesis, 
and  to  elaborate  it  as  a  complete  theory  of  development.  In  his 
first  essay  ('85),  published  before  De  Vries's  paper,  he  went  no  fur- 
ther than  Roux.  "I  believe  that  we  must  accept  the  hypothesis  that 
in  indirect  nuclear  division,  the  formation  of  non-equivalent  halves 
may  take  place  quite  as  readily  as  the  formation  of  equivalent  halves, 
and  that  the  equivalence  or  non-equivalence  of  the  subsequently  pro- 
duced daughter-cells  mu.st  depend  upon  that  of  the  nuclei.  Thus, 
during  ontogeny  a  gradual  transformation  of  the  nuclear  substance 
takes  place,  necessarily  imposed  upon  it,  according  to  certain  laws. 


THE  R  O  UX-  WEISMANN   THE  OR  V   OF  DE  VEL  OPMENT  3  O  5 

by  its  own  nature,  and  such  transformation  is  accompanied  by  a 
gradual  change  in  the  character  of  the  cell-bodies."  ^  In  later  writ- 
ings Weismann  advanced  far  beyond  this,  building  up  an  elaborate 
artificial  system,  which  appears  in  its  final  form  in  the  remarkable 
book  on  the  germ-plasm  ('92).  Accepting  De  Vries's  conception  of 
the  pangens,  he  assumes  a  definite  grouping  of  these  bodies  in  the 
germ-plasm  or  idioplasm  (chromatin),  somewhat  as  in  Nageli's  concep- 
tion. The  pangens  or  biopJiores  are  conceived  to  be  successively  ag- 
gregated in  larger  and  larger  groups;  namely,  (i)  determinants,  which 
are  still  beyond  the  limits  of  microscopical  vision ;  (2)  ids^  which  are 
identified  with  the  visible  chromatin-granules ;  and  (3)  idants,  or 
chromosomes.  The  chromatin  has,  therefore,  a  highly  complex  fixed 
architecture,  which  is  transmitted  from  generation  to  generation,  and 
determines  the  development  of  the  embryo  in  a  definite  and  specific 
manner.  Mitotic  division  is  conceived  as  an  apparatus  which  may^ 
distribute  the  elements  of  the  chromatin  to  the  daughter-nuclei  either 
equally  or  unequally.  In  the  former  case  {^^  hoinceokinesiSj"  mtegral 
or  quantitative  division),  the  resulting  nuclei  remain  precisely  equiva- 
lent. In  the  second  case  {'^ keterokinesis,''  qualitative  or  dijferential 
division),  the  daughter-cells  receive  different  groups  of  chromatin- 
elements,  and  hence  become  differently  modified.  During  ontogeny, 
through  successive  qualitative  divisions,  the  elements  of  the  idioplasm 
Q)X  gcrm-plasvi  (chromatin)  are  gradually  sifted  apart,  and  distributed 
in  a  definite  and  predetermined  manner  to  the  various  parts  of  the 
body.  "  Ontogeny  depends  on  a  gradual  process  of  disintegration  of 
the  id  of  germ-plasm,  which  splits  into  smaller  and  smaller  groups  of 
determinants  in  the  development  of  each  individual.  .  .  .  Finally, 
if  we  neglect  possible  complications,  only  one  kind  of  determinant  re- 
mains in  each  cell,  viz.  that  which  has  to  control  that  particular  cell  or 
group  of  cells.  ...  In  this  cell  it  breaks  up  into  its  constituent  bi- 
ophores,  and  gives  the- cell  its  inherited  specific  character."  ^  Devel- 
opment is,  therefore,  essentially  evolutionary  and  not  epigenetic  ;  ^  its 
point  of  departure  is  a  substance  in  which  all  of  the  adult  characters 
are  represented  by  preformed,  prearranged  germs;  its  course  is  the 
result  of  a  predetermined  harmony  in  the  succession  of  the  qualitative 
divisions  by  which  the  hereditary  substance  is  progressively  disinte- 
grated. In  order  to  account  for  heredity  through  successive  genera- 
tions, Weismann  is  obliged  to  assume  that,  by  means  of  quantitative 
or  integral  division,  a  certain  part  of  the  original  germ-plasm  is  car- 
ried on  unchanged,  and  is  finally  delivered,  with  its  original  architecture 
unaltered,  to  the  germ-nuclei.  The  power  of  regeneration  is  explained, 
in  like  manner,  as  the  result  of  a  transmission  of  unmodified  or  slightly 
modified  germ-plasm  to  those  parts  capable  of  regeneration. 

1  Essay  IV.,  p.  193,  1885.  "  Germ-plasm,  pp.  76,  77.  ^  I.e.,  p.  15. 

X 


3o6 


THEORIES   OF  INHERITANCE  AND   DEVELOPMENT 


E.     Critique  of  the  Roux-Weismann  Theory 


From  a  logical  point  of  view  the  Roux-Weismann  theory  is  unas- 
sailable. Its  fundamental  weakness  is  its  ^//rti^^-metaphysical  char- 
acter, which  indeed  almost  places  it  outside  the  sphere  of  legitimate 
scientific  hypothesis.     Not  a  single  visible  phenomenon  of  cell-divi- 


Fig.  132.  — Half  and  whole  cleavage  in  the  eggs  of  sea-urchins. 
A.  Normal  i6-cell  stage,  showing  the  four  micromeres  above  (from  Driesch,  after  Selenka), 
B.  Half  i6-cell  stage  developed  from  one  blastomere  of  the  2-cell  stage  after  killing  the  other  by 
shaking  (Driesch).  C.  Half  blastula  resulting,  the  dead  blastomere  at  the  right  (Driesch). 
D.  Half-sized  i6-cell  stage  of  Toxopneustes,  viewed  from  the  micromere-pole  (the  eight  lower  cells 
not  shown).  This  embryo,  developed  from  an  isolated  blastomere  of  the  2-cell  stage,  segmented 
like  an  entire  normal  ovum. 

sion  gives  even  a  remote  suggestion  of  qualitative  division.  All  the 
facts,  on  the  contrary,  indicate  that  the  division  of  the  chromatin  is 
carried  out  with  the  most  exact  equality.  The  theory  of  qualita- 
tive division  was  suggested  by  a  totally  different  order  of  phenom- 
ena, and  is  an  explanation  constructed  ad  'hoc.  Roux,  it  is  true,  was 
led  to  the  hypothesis  through  an  examination  of  mitosis ;  but  it  is 


CRITIQUE    OF   THE  ROUX-WEISMANN   THEORY 


1>07 


safe  to  say  that  he  would  never  have  maintained  in  the  same  breath 
that  mitosis  is  expressly  designed  for  quantitative  and  also  for  qual- 
itative division,  had  he  fixed  his  attention  on  the  actual  phenomena 
of  mitosis^alone.  The  hypothesis  is  in  fact  as  complete  an  a  priori 
assumption  as  any  that  the  history  of  scholasticism  can  show,  and 
every  fact  opposed  to  it  has  been  met  by  equally  baseless  subsidiary 
hypotheses,  which,  like  their  principal,  relate  to  matters  beyond  the 
reach  of  observation. 

Such  an  hypothesis  cannot  be  actually  overturned  by  an  appeal  to 
fact.     When,  however,  we  make  such  an  appeal,  the  improbability  of 


Fig.  133.  —  Normal  and  dwarf  gastrulas  of  Amphioxus. 
A.  Normal  gastrula.     B.  Half-sized  dwarf,  from  an  isolated  blastomere   of  the  2-cell  stage. 
C.  Quarter-sized  dwarf,  from  an  isolated  blastomere  of  the  4-cell  stage. 


the  hypothesis  becomes  so  great  that  it  loses  all  semblance  of  reality. 
It  is  rather  remarkable  that  Roux  himself  led  the  way  in  this  direc- 
tion. In  the  course  of  his  observations  on  the  development  of  a  half- 
embryo  from  one  of  the  blastomeres  of  the  two-cell  stage  he  determined 
the  significant  fact  that  the  half-embryo  afterwards  rege^terated  the 
missing  Jialf,  a7td  gave  rise  to  a  complete  embryo.  Essentially  the 
same  result  was  reached  by  later  observers,  both  in  the  frog  (Endres, 
Walter,  Morgan)  and  in  a  number  of  other  animals,  with  the  impor- 
tant addition  that  the  half-formation  is  sometimes  characteristic  of 
only  the  earliest  stages  and  may  be  entirely  suppressed.  In  189 1 
Driesch  was  able  to  follow  out  the  development  of  isolated  blasto- 


3o8 


THEORIES   OF  INHERITANCE  AND  DEVELOPMENT 


meres  of  sea-urchin  eggs  separated  by  shaking  to  pieces  the  two- 
cell  and  four-cell  stages.  Blastomeres  thus  isolated  segment  as  if 
still  forming  part  of  an  entire  larva,  and  give  rise  to  a  half-  (or  quar- 
ter-) blastula  (Fig.  132).    The  opening  soon  closes,  however,  to  form  a 


Fig.  134.  —  Dwarf  and  double  ^mhxsos  oi  Aiuphioxus. 
A.  Isolated  blastomere  of  the  2-cell  stage  segmenting  like  an  entire  egg  (cf.  Fig,  123,/)). 
B.  Twin  gastrulas  from  a  single  egg.     C.  Double  cleavage  resulting  from  the  partial  separation, 
by  shaking,  of  the  blastomeres  of  the  2-cell  stage.    D.  E.  F.  Double  gastrulas  arising  from  such 
forms  as  the  last. 


small  complete  blastula,  and  the  resulting  gastrula  and  Pluteus  larva 
is  a  perfectly  formed  dwarf  of  only  half  (or  quarter)  the  normal  size. 
Incompletely  separated  blastomeres  gave  rise  to  double  embryos  like 
the  Siamese  twins.  Shortly  afterwards  the  writer  obtained  similar 
result  in  the  case  of  AmpJiioxus,  but  here  the  isolated  blastomere  seg- 


CRITIQUE    OF   THE  ROUX-WEISMANN   THEORY 


309 


tncnts  from  the  begin?iing  like  an  entire  ovum  of  diminisJied size  (Figs. 
133,  124).  The  same  result  has  since  been  reached  by  Morgan  in  the 
teleost  fishes,  and  by  Zoja  in  the  medusae.  The  last-named  experi- 
menter w^  able  to  obtain  perfect  embryos  not  only  from  blasto- 
meres  of  the  two-cell  and  four-cell  stages,  but  from  eight-cell  and 
even  from  sixteen-cell  stages,  the  dwarfs  in  the  last  case  being  but 
^ig  the  normal  size ! 

These  experiments  gave  a  fatal  blow  to  the  entire  Roux-Weismann 
theory  ;  for  the  results  showed  that  the  cleavage  of  the  ovum  does  not 
in  these  cases  sunder  different  materials,  either  nuclear  or  cytoplasmic, 
but  only  splits  it  up  into  a  number  of  similar  parts,  each  of  which 
may  give  rise  to  an  entire  body  of  diminished  size. 

The  theory  of  qualitative  nuclear  division   has   been  practically 


Fig.  135.  —  Modification  of  cleavage  in  sea-urchin  eggs  by  pressure. 
A.  Normal  8-cell  stage  of  Toxopveustes.     B.  Eight-cell  stage  of  Echinus  segmenting  under 
pressure.     Both  forms  produce  normal  Plutei. 

disproved  in  another  way  by  Driesch,  through  the  pressure-experi- 
ments already  mentioned  at  p.  275.  Following  the  earlier  experiments 
of  Pfliiger  and  Roux  on  the  frog's  ^^'g,  Driesch  subjected  segmenting 
eggs  of  the  sea-urchin  to  pressure,  and  thus  obtained  flat  plates  of 
cells  in  which  the  arrangement  of  the  nuclei  differed  totally  from  the 
normal  (Fig.  134) ;  yet  such  eggs  when  released  from  pressure  continue 
to  segment,  witJioiit  rearrangement  of  the  nuclei,  and  give  rise  to  per- 
fectly normal  larvae.  I  have  repeated  these  experiments  not  only  with 
sea-urchin  eggs,  but  also  with  those  of  an  annelid  {Nereis),  which  yield 
a  very  convincing  result,  since  in  this  case  the  histological  differentia- 
tion of  the  cells  appears  very  early.  In  the  normal  development  of 
this  animal  the  archenteron  arises  from  four  large  cells  or  macro- 
meres  (entomeres),  which  remain  after  the  successive  formation  of 
three  quartets  of  micromeres  (ectomeres)  and  the  parent-cell  of  the 


lO 


THEORIES   OF  INHERITANCE  AND  DEVELOPMENT 


mesoblast.  After  the  primary  differentiation  of  the  germ-layers  the 
four  entomeres  do  not  divide  again  until  a  very  late  period  (free- 
swimming  trochophore),  and  their  substance  always  retains  a  charac- 
teristic appearance,  differing  from  that  of  the  other  blastomeres  in 
its  pale  non-granular  character  and  in  the  presence  of  large  oil-drops. 


Fig.  136.  —  Modification  of  cleavage  by  pressure  in  Nereis. 
A.  B.  Normal  4-  and  8-cell  stages.     C.  Normal  trochophore  larva  resulting,  with  four  entoderm- 
cells.     D.  Eight-cell  stage  arising  from  an  egg  flattened  by  pressure  ;  such  eggs  give  rise  to  trocho- 
phores  with  eight  instead  of  four  entoderm-cells.     Numerals  designate  the  successive  cleavages. 


If  unsegmented  eggs  be  subjected  to  pressure,  as  in  Driesch's  echino- 
derm  experiments,  they  segment  in  a  flat  plate,  all  of  the  cleavages 
being  vertical.  In  this  way  are  formed  eight-celled  plates  in  which  all 
of  the  cells  contain  oil-drops  (Fig.  136,  D).  If  they  are  now  released 
from  the  pressure,  each  of  the  cells  divides  in  a  plane  approximately 
horizontal,  a  smaller  granular  micromere  being  formed  above,  leaving 


ON   THE  JVA  TURE   AND    CA  USES   OF  DIFFERENTIA  TION       3  1 1 

below  a  larger  clear  macromere  in  which  the  oil-drops  remain. 
The  sixteen-cell  stage,  therefore,  consists  of  eight  deutoplasm-laden 
macromeres  and  eight  protoplasmic  micromeres  (instead  of  four 
macromer^s  and  twelve  micromeres,  as  in  the  usual  development). 
These  embryos  developed  into  free-swimming  trochophores  contain- 
ing eight  instead  of  four  macromeres,  which  have  the  typical  clear 
protoplasm  containing  oil-drops.  In  this  case  there  can  be  no  doubt 
whatever  that  four  of  the  entoblastic  nuclei  were  normally  destined 
for  the  first  quartet  of  micromeres  (Fig.  136,  B),  from  which  arise  the 
apical  ganglia  and  the  prototroch.  Under  the  conditions  of  the 
experiment,  however,  they  have  given  rise  to  the  nuclei  of  cells 
which  differ  in  no  wise  from  the  other  entoderm-cells.  Even  in  a 
highly  differentiated  type  of  cleavage,  therefore,  the  nuclei  of  the 
segmenting  ^gg  are  not  specifically  different,  as  the  Roux-Weismann 
hypothesis  demands,  but  contain  the  same  materials  even  in  cells  that 
undergo  the  most  diverse  subsequent  fate.  But  there  is,  furthermore, 
very  strong  reason  for  believing  that  this  may  be  true  in  later  stages 
as  well,  as  Kolliker  insisted  in  opposition  to  Weismann  as  early  as 
1886,  and  as  has  been  urged  by  many  subsequent  writers.  The  strong- 
est evidence  in  this  direction  is  afforded  by  the  facts  of  regeneration ; 
and  many  cases  are  known  —  for  instance  among  the  hydroids  and  the 
plants  —  in  which  even  a  small  fragment  of  the  body  is  able  to  repro- 
duce the  whole.  It  is  true  that  the  power  of  regeneration  is  always 
limited  to  a  greater  or  less  extent  according  to  the  species.  But  there 
is  no  evidence  whatever  that  such  limitation  arises  through  specifica- 
tion of  the  nuclei  by  qualitative  division,  and,  as  will  appear  beyond, 
its  explanation  is  probably  to  be  sought  in  a  very  different  direction. 


F.     On  the  Nature  and  Causes  of  Differentiation 

We  have  now  cleared  the  ground  for  a  restatement  of  the  prob- 
lem of  development,  and  an  examination  of  the  views  opposed  to  the 
Roux-Weismann  theory.  After  discarding  the  hypothesis  of  quali- 
tative division  the  problem  confronts  us  in  the  following  form.  If 
chromatin  be  the  idioplasm  in  which  inheres  the  sum-total  of  heredi- 
tary forces,  and  if  it  be  equally  distributed  at  every  cell-division,  how 
can  its  mode  of  action  so  vary  in  different  cells  as  to  cause  diversity 
of  structure,  i.e.  differentiation?  It  is  perfectly  certain  that  differen- 
tiation is  an  actual  progressive  transformation  of  the  egg-substance 
involving  both  physical  and  chemical  changes,  occurring  in  a  definite 
order,  and  showing  a  definite  distribution  in  the  regions  of  the  ^gg. 
These  changes  are  sooner  or  later  accompanied  by  the  cleavage 
of    the    Qgg    into    cells    whose    boundaries    may    sharply    mark    the 


312  THEORIES   OF  INHERITANCE   AND   DEVELOPMENT 

areas  of  differentiation.  What  gives  these  cells  their  specific  char- 
acter? Why,  in  the  four-cell  stage  of  an  annelid  ^gg,  should  the 
four  cells  contribute  equally  to  the  formation  of  the  alimentary  canal 
and  the  cephalic  nervous  system,  while  only  one  of  them  (the  left- 
hand  posterior)  gives  rise  to  the  nervous  system  of  the  trunk-region 
and  to  the  muscles,  connective  tissues,  and  the  germ-cells?  (Figs.  122, 
137,  B).  There  cannot  be  a  fixed  and  necessary  relation  of  cause 
and  effect  between  the  various  regions  of  the  ^gg  which  these  blas- 
tomeres  represent  and  the  adult  parts  arising  from  them ;  for,  as  we 
have  seen,  these  relations  may  be  artificially  altered.  A  portion  of 
the  Qg'g  which  under  normal  conditions  would  give  rise  to  only  a 
fragment  of  the  body  will,  if  split  off  from  the  rest,  give  rise  to  an 
entire  body  of  diminished  size.  What  then  determines  the  history 
of  such  a  portion  ?  What  influence  moulds  it  now  into  an  entire 
body,  now  into  a  part  of  a  body  ? 

De  Vries,  in  his  remarkable  essay  on  Iiitracelhdar  Pangenesis 
('89),  endeavoured  to  cut  this  Gordian  knot  by  assuming  that  the 
character  of  each  cell  is  determined  by  pangens  that  migrate  from 
the  nucleus  into  the  cytoplasm,  and,  there  becoming  active,  set  up 
specific  changes  and  determine  the  character  of  the  cell,  this  way 
or  that,  according  to  their  nature.  But  what  influence  guides  the 
migration  of  the  pangens,  and  so  correlates  the  operations  of  devel- 
opment ?  Both  Driesch  and  Oscar  Hertwig  have  attempted  to 
answer  this  question,  though  the  first-named  author  does  not  commit 
himself  to  the  pangen  hypothesis.  These  writers  have  maintained 
that  the  particular  mode  of  development  in  a  given  region  or  blasto- 
mere  of  the  ^gg  is  a  result  of  its  relation  to  the  remainder  of  the  inass^ 
i.e.  a  product  of  what  may  be  called  the  intra-embryonic  environ- 
ment. Both  at  first  assumed  not  only  that  the  nuclei  are  equivalent, 
but  also  that  the  cytoplasmic  regions  of  the  Qgg  are  isotropic,  i.e. 
primarily  composed  of  the  same  materials  and  equivalent  in  struct- 
ure. Hertwig  insisted  that  the  organism  develops  as  a  whole  as  the 
result  of  a  formative  power  pervading  the  entire  mass  ;  that  differen- 
tiation is  but  an  expression  of  this  power  acting  at  particular  points ; 
and  that  the  development  of  each  part  is,  therefore,  dependent  on 
that  of  the  whole. ^  "According  to  my  conception,"  said  Hertwig, 
"  each  of  the  first  two  blastomeres  contains  the  formative  and  differ- 
entiating forces  not  simply  for  the  production  of  a  half-body,  but  for 
the  entire  organism  ;  the  left  blastomere  develops  into  the  left  half 
of  the  body  only  because  it  is  placed  in  relation  to  a  right  blasto- 
mere." ^     Again,    in    a    later    paper:  —  *'The    Qgg    is    a    specifically 

1  Whitman  had  strongly  urged  this  view  several  years  before,  and  a  nearly  similar  concep- 
tion lay  at  the  bottom  of  Herbert  Spencer's  theory  of  development.     Cf.  pp.  41,  293. 

2  '92,  I,  p.  481. 


ON   THE  NATURE   AND    CAUSES   OF  DIFFERENTIATION       313 

organized  elementary  organism  that  develops  epigenetically  by 
breaking  up  into  cells  and  their  subsequent  differentiation.  Since 
every  elementary  part  {i.e.  cell)  arises  through  the  division  of  the 
germ,  or^'fertilized  Q.gg,  it  contains  also  the  germ  of  the  whole/  but 
during  the  process  of  development  it  becomes  ever  more  precisely 
differentiated  and  determined  by  the  formation  of  cytoplasmic  prod- 
ucts according  to  its  position  with  reference  to  the  entire  organism 
(blastula,  gastrula,  etc)."^ 

Driesch  expressed  the  same  view  with  great  clearness  and  pre- 
cision shortly  after  Hertwig  :  —  "The  fragments  {i.e.  cells)  produced 
by  cleavage  are  completely  equivalent  or  indifferent."     *'The  blasto- 


Fig.  137.  —  Diagrams  contrasting  the  value  of  the  blastomeres  in  polyclades  and  annelids. 

A.  Plan  of  cleavage  in  the  polyciade  egg  (constructed  from  the  figures  of  Lang).  B.  Corre- 
sponding plan  of  the  annelid  egg.  In  both  cases  the  ectoblast  is  unshaded,  with  the  exception  of 
X :  the  mesoblast  is  ruled  in  vertical  lines  and  the  entoblast  in  horizontal.  In  both,  three  succes- 
sive quartets  of  micromeres  are  budded  forth  from  the  four  primary  cells  A.  B.  C.  D.  In  the 
polyciade  the  first  quartet  is  ectoblastic,  the  second  and  third  mesoblastic.  In  the  annelid  all  three 
quartets  are  ectoblastic,  while  the  mesoblast  {M)  arises  from  the  posterior  cell  of  a  fourth  quartet 
of  which  the  remaining  three  are  entoblastic. 


meres  of  the  sea-urchin  are  to  be  regarded  as  forming  a  uniform 
material,  and  they  may  be  thrown  about,  like  balls  in  a  pile,  without 
in  the  least  degree  impairing  thereby  the  normal  power  of  develop- 
ment." ^  ''The  relative  position  of  a  blastoniere  in  the  whole  de- 
termines in  general  ivhat  develops  from  it;  if  its  position  be  cJianged, 
it  gives  rise  to  something  different ;  in  other  words,  its  prospective 
value  is  a  function  of  its  position.""  ^ 

This  conclusion  undoubtedly  expresses  a  part  of  the  truth,  though, 
as  will  presently  appear,  it  is  too  extreme.     The  relation  of  the  part 

^  That  is,  in  the  specifically  organized  chromatin  within  the  nucleus. 


93.  P-  79: 


^  Studien  IV.  p. 


"*  Studien  IV.  p.  39. 


314 


THEORIES   OF  INHERITANCE   AND  DEVELOPMENT 


to  the  whole  must  not,  however,  be  conceived  as  a  merely  geometri- 
cal or  mechanical  one ;  for,  in  different  species  of  eggs,  blastomeres 
may  exactly  correspond  in  origin  and  relative  position,  yet  have 
entirely  different  morphological  value.     This  is  strikingly  shown  by 


Fig.  138.  —  Partial  larvae  of  the  ctenophore  Beroe.  [Driesch  and  MORGAN.] 
A.  Half  i6-cell  stage,  from  an  isolated  blastomere.  B.  Resulting  larva,  with  four  rows  of  swim- 
ming plates  and  three  gastric  pouches.  C.  One-fourth  i6-cell  stage,  from  an  isolated  blastomere. 
D.  Resulting  larva  with  two  rows  of  plates  and  two  gastric  pouches.  E.  Defective  larva,  with  six 
rows  of  plates  and  three  gastric  pouches,  from  a  nucleated  fragment  of  an  unsegmented  ^gg. 
F.  Similar  larva  with  five  rows  of  plates,  from  above. 


a  comparison  of  the  polyclade  Qgg  with  that  of  the  annelid  or 
gasteropod  (Fig.  137).  In  both  cases  three  quartets  of  micromeres 
are  successively  budded  off  from  the  four  cells  of  the  four-cell 
stage  in  exactly  the  same  manner.  The  first  quartet  in  both  gives 
rise  to  ectoderm.     Beyond  this  point,  however,  the  agreement  ceases ; 


ON   THE  NATURE   AND    CAUSES   OF  DIFFERENTIATION       315 

for  the  second  and  third  quartets  form  mesoblast  in  the  polyclade, 
but  ectoblast  in  the  annelid  and  gasteropod !  In  the  latter  forms 
the  mesoblast  lies  in  a  single  cell  belonging  to  a  fourth  quartet  of 
which  the  other  three  cells  form  entoblast.  This  shows  conclusively 
that  the  relation  of  the  part  to  the  whole  is  of  an  exceedingly  subtle 
character,  and  that  the  nature  of  the  individual  blastomere  depends, 
not  merely  upon  its  geometrical  position,  but  upon  its  physiological 
relation  to  tJie  inJieritcd  organization  of  which  it  forms  a  part. 

Meanwhile,  and  subsequently,  however,  facts  were  determined 
that  threw  doubts  on  the  hypothesis  of  cytoplasmic  isotropy  and 
led  Driesch  to  a  profound  modification  of  his  views,  and  in  a 
measure  rehabilitated  the  theory  of  cytoplasmic  localization.  Whit- 
man, Morgan,  and  Driesch  himself  showed  that  the  cytoplasm  of 
the  echinoderm  ^^'g  is  not  strictly  isotropic,  as  Hertwig  assumed ; 
for  the  ovum  possesses  a  polarity  predetermined  before  cleavage 
begins,  as  proved  by  the  fact  that  a  group  of  small  cells  or  micro- 
meres  always  arises  at  a  certain  point  which  may  be  precisely  located 
before  cleavage  by  reference  to  the  eccentricity  of  the  first  cleavage- 
nucleus.^  Experiments  on  the  eggs  of  other  animals  proved  that  the 
predetermination  of  the  cytoplasmic  regions  may  be  more  extensive. 
In  the  ^^g  of  the  ctenophore,  for  example,  Driesch  and  Morgan 
('95),  confirming  the  earlier  observations  of  Chun,  proved  that  an 
isolated  blastomere  of  the  two-  or  four-cell  stage  gives  rise  not  to  a 
whole  dwarf  body,  but  to  a  half-  or  quarter-body,  as  Roux  had 
observed  in  the  frog^  (Fig.  135,  A-D).  But,  more  than  this,  these 
experimenters  made  the  interesting  discovery  that  if  a  part  of  the 
cytoplasm  of  an  imscgincnted  ctenophore-egg  were  removed,  the 
remainder  gave  rise  to  an  incomplete  larva,  sJwzving  certain  defects 
wJiicJi  7'epresent  the  portions  removed  (Fig.  138^  E,  F).  Again, 
Crampton  found  that  in  case  of  the  marine  gasteropod  Ilyajtassa, 
isolated  blastomeres  of  two-cell  or  four-cell  stages  segmented  exactly 
as  if  forming  part  of  an  entire  embryo  and  gave  rise  \.o  fragments  of 
a  larva,  not  to  complete  dwarfs,  as  in  the  echinoderm  (Fig.  139). 

These  results  demonstrate  that  the  ovum  may  show  a  high  degree 
of  cytoplasmic  localization  and  that  in  such  cases  cleavage  may  be  in 
fact  a  mosaic-work,  as  Roux  maintained  in  case  of  the  frog.  But 
they  also  show  that  the  localization,  and  the  resulting  mosaic-like 
cleavage,  is  not  determined  by  specific  differences  in  the  nuclei ;  for 
in  the  ctenophore  the  fragment  of  an  nnsegmented  Q,gg,  though  con- 
taining an  entire  nucleus,  gives  rise  to  a  defective  larva,  and  in  Nereis 
the  nuclei  may  be  shifted  about  at  will  without  altering  the  develop- 

1  Cf.  Fig.  77. 

-  The  larva  is,  however,  not  a  strict  partial  one,  since  it  makes  an  abortive  attempt  to 
form  the  normal  number  of  gastric  pouches. 


3i6 


THEORIES    OF  INHERITANCE  AND   DEVELOPMENT 


ment.  And  if  the  germinal  localization  is  not  directly  determined  by 
the  nuclei  it  must  here  be  determined  by  a  pre-organization  of  the 
cytoplasmic  substance.  How  is  this  result  to  be  reconciled  with  the 
experiments  on  Amphioxiis  and  the  echinoderms,  and  with  the  more 
o-eneral  conclusion  that  the  ultimate  determining  causes  of  differentia- 


C  D  ^— ^       E 

Fig.  139. —  Partial  development  of  isolated  blastomeres  of  the  gasteropod  egg,  Ilyanasm. 
[Cramfton.] 

A.  Normal  8-cell  stage.  B.  Normal  i6-cell  stage.  C.  Half  8-cell  stage,  from  isolated  blasto- 
mere  of  the  2-cell  stage.  D.  Half  i2-cei!  stage  succeeding.  E.  Two  stages  in  the  cleavage  of  an 
isolated  blastomere  of  the  4-cell  stage;  above  a  one-fourth  8-cell  stage,  below  a  one-fourth  i6-cell 
stage. 

tion  are  to  be  sought  in  the  nucleus  .'*  The  difficulty  at  once  disap- 
pears when  we  recall  that  development  and  differentiation  do  not  in 
any  proper  sense  first  begin  with  the  cleavage  of  the  ovum,  but  long 
before  this,  during  its  ovarian  history.  The  primary  differentiations 
thus  established  in  the  cytoplasm  form  the  immediate  conditions 
to  which  the  later  development  must  conform  ;   and  the  difference 


ON   THE  NATURE   AND    CAUSES   OF  DIFFERENTIATION       317 

between  Ampliioxns  on  the  one  hand,  and  the  snail  or  ctenophore 
on  the  other,  simply  means,  I  think,  that  the  initial  differentiation  is 
less  extensive  or  less  firmly  established  in  the  one  than  in  the  other. 

We  thu«-  arrive  at  the  central  point  of  my  own  conception  of  devel- 
opment, and  of  Driesch's  later  views;  which  were  developed  in  a  most 
able  and  suggestive  though  somewhat  abstruse  manner  in  his  Analy- 
tische  Theorie  der  organiscJien  EntzvickliLUg  {^<^d^\  and  slightly  modified 
in  a  later  paper  published  jointly  with  Morgan  ('95,  2).  The  gist  of 
Driesch's  theory  is  as  follows.  All  the  nuclei  are  equivalent,  and  all 
contain  the  same  idioplasm  equally  distributed  to  them  by  mitotic 
division.  Through  the  influence  of  this  idioplasm  the  cytoplasm  of 
the  ^gg,  or  of  the  blastomeres  derived  from  it,  undergoes  specific  and 
progressive  changes,  each  change  reacting  upon  the  nucleus  and  thus 
inciting  a  new  change.  These  changes  differ  in  different  regions  of 
the  ^gg  because  of  pre-existing  differences,  chemical  and  physical, 
in  the  cytoplasmic  structure  ;  and  these  form  the  conditions  ("Form- 
bildungsfaktoren  ")  under  which  the  idioplasm  operates.  Some  of 
these  conditions  are  purely  mechanical,  such  as  the  shape  of  the 
ovum,  the  distribution  of  deutoplasm,  and  the  like.  Others,  and 
probably  the  more  important,  are  far  more  subtle,  such  as  the  distri- 
bution of  different  chemical  substances  in  the  cytoplasm,  and  the 
unknown  polarities  of  the  cytoplasmic  molecules. 

A  nearly  related  conception  was  developed  with  admirable  clear- 
ness by  Oscar  Hertwig  ('94)  nearly  at  the  same  time.  Both 
Driesch  and  Hertwig  thus  retreated  in  a  measure  towards  the 
theory  of  germinal  localization  in  the  cytoplasm,  which  both  had  at 
first  rejected;  but  only  to  a  middle  ground  which  lies  between  the 
two  extremes  of  the  strict  predestination  theory  and  the  theory  of 
cytoplasmic  isotropy.  For  these  writers  now  maintain  that  the  initial 
cytoplasmic  localization  of  the  formative  conditions  is  of  limited  extent 
and  determines  only  the  earlier  steps  of  development.  With  each 
forward  step  new  conditions  (chemical  differentiations  and  the  like) 
are  established  which  form  the  basis  for  the  ensuing  change,  and  so 
on  in  ever-increasing  complexity.  This  view  is  substantially  the  same 
as  that  which  I  have  myself  urged  in  several  earlier  works,  and  I  have 
pointed  out  how  it  enables  us  to  reconcile  the  apparent  contradiction 
between  the  partial  development  of  isolated  blastomeres  of  such 
forms  as  the  ctenophore,  on  the  one  hand,  with  the  total  development 
of  such  forms  as  AmpJiioxus  or  the  echinoderm,  on  the  other.  In  the 
latter  case  we  may  suppose  the  cytoplasmic  differentiation  to  be  but 
feebly  established  at  the  beginning,  and  the  blastomeres  remain  for  a 
time  in  a  plastic  state,  which  enables  them  on  isolation  to  revert  to 
the  condition  of  the  original  entire  ovum.  In  the  former  case  the 
initial  differentiation  is  more  extensive  or  more  rigidly  fixed,  so  that 


3i8 


THEORIES   OF  INHERITANCE   AND   DEVELOPMENT 


the  development  of  the  blastomere  is  from  the  begmning  hemmed 
in  by  the  cytoplasmic  conditions,  and  its  powers  are  correspondingly 
limited.  In  such  cases  the  cleavage  may  exhibit  more  or  less  of 
a  mosaic-like  character,  and  the  theory  of  cytoplasmic  localization 
acquires  a  real  meaning  and  value. 

That  we  are  here  approaching  the  true  explanation  is  indicated  by 


Fig.  140.  —  Double  embryos  of  frog  developed  fiom  eggs  inverted  when  in  the  2-cell  stage. 
[O.   SCHULTZE.] 

A.  Twins  with  heads  turned  in  opposite  directions.  D.  Twins  united  back  to  back.  C.  Twins 
united  by  their  ventral  sides.     D.  Double-headed  tadpole. 


certain  very  remarkable  and  interesting  experiments  on  the  frog's 
^ZZ  which  prove  that  each  of  the  first  two  blastomeres  may  give  rise 
either  to  a  half-embryo  or  to  a  whole  embryo  of  half  size,  according 
to  circumstances,  and  which  indicate,  furthermore,  that  these  circum- 
stances lie  in  a  measure  in  the  arrangement  of  the  cytoplasmic 
materials.     This  most  important  result,  which  we  owe  especially  to 


ON   THE  NATURE  AND    CAUSES   OF  DIFFERENTIATION       319 

Morgan,^  was  reached  in  the  following  manner.  Born  had  shown,  in 
1885,  that  if  frogs'  eggs  be  fastened  in  an  abnormal  position, — e.g. 
upside  down,  or  on  the  side,  —  a  rearrangement  of  the  egg-material 
takes  place,  the  heavier  deutoplasm  sinking  towards  the  lower  side, 
while  the  nucleus  and  protoplasm  rise.  A  new  axis  is  tJms  established 
in  the  egg,  which  has  the  same  relation  to  the  body-axes  as  in  the 
ordinary  development  (though  the  pigment  retains  its  original  arrange- 
ment). This  proves  that  in  eggs  of  this  character  (telolecithal)  the 
distribution  cf  deutoplasm,  or  conversely  of  protoplasm,  is  one  of  the 
primary  formative  conditions  of  the  cytoplasm ;  and  the  significant 
fact  is  that  by  ai'tificially  changing  this  distribntion  the  axis  of  the 
embryo  is  shifted.  Oscar  Schultze  ('94)  discovered  that  if  the  ^gg  be 
turned  upside  down  when  in  the  two-cell  stage,  a  whole  embryo  (or 
half  of  a  double  embryo)  might  arise  from  each  blastomere  instead 
of  a  half-embryo  as  in  the  normal  development,  and  that  the  axes  of 
these  embryos  show  no  constant  relation  to  one  another  (Fig.  140). 
Morgan  ('95,3)  added  the  important  discovery  that  either  a  half- 
embryo  or  a  whole  half-sized  dwarf  might  be  formed,  according  to  the 
position  of  the  blastomere.  If,  after  destruction  of  one  blastomere,  the 
other  be  allowed  to  remain  in  its  normal  position,  a  half-embryo  always 
results,^  precisely  as  described  by  Roux.  If,  on  the  other  hand,  the 
blastomere  be  inverted,  it  may  give  rise  either  to  a  half-embryo  ^  or  to 
a  whole  dwarf.'*  Morgan  therefore  concluded  that  the  production  of 
whole  embryos  by  the  inverted  blastomeres  was,  in  part  at  least,  due 
to  a  rearrangement  or  rotation  of  the  egg-materials  under  the  influence 
of  gravity,  the  blastomere  thus  returning,  as  it  were,  to  a  state  of 
equilibrium  like  that  of  an  entire  ovum. 

This  beautiful  experiment  gives  most  conclusive  evidence  that  each 
of  the  two  blastomeres  contains  all  the  materials,  nuclear  and  cyto- 
plasmic, necessary  for  the  formation  of  a  whole  body  ;  and  that  these 
materials  may  be  used  to  build  a  whole  body  or  half-body,  according 
to  the  grouping  that  they  assume.  After  the  first  cleavage  takes 
place,  each  blastomere  is  set,  as  it  were,  for  a  half-development,  but 
not  so  firmly  that  a  rearrangement  is  excluded. 

I  have  reached  a  nearly  related  result  in  the  case  both  of  Amphi- 
oxus  and  the  echinoderms.  In  Amphioxus  the  isolated  blastomere 
usually  segments  like  an  entire  ovum  of  diminished  size.  This  is, 
however,  not  invariable,  for  a  certain  proportion  of  the  blastomeres 
show  a  more  or  less  marked  tendency  to  divide  as  if  still  forming  part 
of  an  entire  embryo.  The  sea-urchin  Toxopnenstes  reverses  this 
rule,  for  the  isolated  blastomere  of  the  two-cell  stage  usually  shows  a 
perfectly  typical  half-cleavage,  as  described  by  Driesch,  but  in  rare 

1  Anat.  Anz.,  X.  19,  1895.  ^  Three  cases. 

'■^  Eleven  cases  observed.  *  Nine  cases  observed. 


320  THEORIES   OF  INHERITANCE  AND  DEVELOPMENT 

cases  it  may  segment  like  an  entire  ovum  of  half-size  (Fig.  132,  D)  and 
give  rise  to  an  entire  blastula.^  We  may  interpret  this  to  mean  that 
in  Amphioxus  the  differentiation  of  the  cytoplasmic  substance  is  at 
first  very  slight,  or  readily  alterable,  so  that  the  isolated  blastomere, 
as  a  rule,  reverts  at  once  to  the  condition  of  the  entire  ovum.  In  the 
sea-urchin,  the  initial  differentiations  are  more  extensive  or  more 
firmly  established,  so  that  only  exceptionally  can  they  be  altered.  In 
the  snail  we  have  the  opposite  extreme  to  AmpJiioxus,  the  cytoplasmic 
conditions  having  been  so  firmly  established  that  they  cannot  be 
altered,  and  the  development  must,  from  the  outset,  proceed  within 
the  limits  thus  set  up. 

Through  this  conclusion  we  reconcile,  as  I  believe,  the  theories  of 
cytoplasmic  localization  and  mosaic  development  with  the  hypothesis 
of  cytoplasmic  isotropy.  Primarily  the  egg-cytoplasm  is  isotropic  in 
the  sense  that  its  various  regions  stand  in  no  fixed  and  necessary  rela- 
tion with  the  parts  to  which  they  respectively  give  rise.  Secondarily, 
however,  it  may  undergo  differentiations  through  which  it  acquires  a 
definite  regional  predetermination  which  becomes  ever  more  firmly 
established  as  development  advances.  This  process  does  not,  how- 
ever, begin  at  the  same  time,  or  proceed  at  the  same  rate  in  all  eggs. 
Hence  the  eggs  of  different  animals  may  vary  widely  in  this  regard 
at  the  time  cleavage  begins,  and  hence  may  differ  as  widely  in  their 
power  of  response  to  changed  conditions. 

The  origin  of  the  cytoplasmic  differentiations  existing  at  the  be- 
ginning of  cleavage  has  already  been  considered  (p.  285).  If  the 
conclusions  there  reached  be  placed  beside  the  above,  we  reach  the 
following  conception.  The  primary  determining  cause  of  develop- 
ment lies  in  the  nucleus,  which  operates  by  setting  up  a  continuous 
series  of  specific  metabolic  changes  in  the  cytoplasm.  This  process 
begins  during  ovarian  growth,  establishing  the  external  form  of  the 
Qg^,  its  primary  polarity,  and  the  distribution  of  substances  within  it. 
The  cytoplasmic  differentiations  thus  set  up  form  as  it  were  a  frame- 
work within  which  the  subsequent  operations  take  place,  in  a  more 
or  less  fixed  course,  and  which  itself  becomes  ever  more  complex  as 
development  goes  forward.  If  the  cytoplasmic  conditions  be  artifi- 
cially altered  by  isolation  or  other  disturbance  of  the  blastomeres,  a 
readjustment  may  take  place  and  development  may  be  correspond- 
ingly altered.  Whether  such  a  readjustment  is  possible,  depends  on 
secondary  factors  —  the  extent  of  the  primary  differentiations,  the 
physical  consistency  of  the  egg-substance,  the  susceptibility  of  the 
protoplasm  to  injury,  and  doubtless  a  multitude  of  others. 

1  I  have  observed  this  only  twice.  In  both  cases  the  cleavage  up  to  the  sixteen-cell  stage 
was  exactly  like  that  of  the  entire  egg  except  that  the  micromeres  were  relatively  larger,  as 
shown  in  the  figure. 


THE  NUCLEUS  IN  LATER  DEVELOPMENT  321 


G.  The  Nucleus  in  Later  Development 

The  fo*'egoing  conception,  as  far  as  it  goes,  gives  at  least  an  in- 
telligible view  of  the  more  general  features  of  early  development  and 
in  a  measure  harmonizes  the  apparently  conflicting  results  of  experi- 
ment on  various  forms.  But  there  are  a  very  large  number  of  facts 
relating  especially  to  the  later  stages  of  differentiation,  which  it 
leaves  wholly  unexplained,  and  which  indicate  that  the  nucleus  as 
well  as  the  cytoplasm  may  undergo  progressive  changes  of  its  sub- 
stance. It  has  been  assumed  by  most  critics  of  the  Roux-Weismann 
theory  that  all  of  the  nuclei  of  the  body  contain  the  same  idioplasm, 
and  that  each  therefore,  in  Hertwig's  words,  contains  the  germ  of  the 
whole.  There  are,  however,  a  multitude  of  well-known  facts  which 
cannot  be  explained,  even  approximately,  under  this  assumption. 
The  power  of  a  single  cell  to  produce  the  entire  body  is  in  general 
limited  to  the  earliest  stages  of  cleavage,  rapidly  diminishes,  and  as 
a  rule  soon  disappears  entirely.  When  once  the  germ-layers  have 
been  definitely  separated,  they  lose  entirely  the  power  to  regenerate 
one  another  save  in  a  few  exceptional  cases.  In  asexual  reproduction, 
in  the  regeneration  of  lost  parts,  in  the  formation  of  morbid  growths, 
each  tissue  is  in  general  able  to  reproduce  only  a  tissue  of  its  own  or 
a  nearly  related  kind.  Transplanted  or  transposed  groups  of  cells 
(grafts  and  the  like)  retain  more  or  less  completely  their  autonomy 
and  vary  only  within  certain  well-defined  limits,  despite  their  change 
of  environment.  All  of  these  statements  are,  it  is  true,  subject  to 
exception  ;  yet  the  facts  afford  an  overwhelming  demonstration  that 
differentiated  cells  possess  a  specific  character,  that  their  power  of 
development  and  adaptability  to  changed  conditions  becomes  in  a 
greater  or  less  degree  limited  with  the  progress  of  development. 
How  can  we  explain  this  progressive  specification  of  the  tissue-cells 
and  how  interpret  the  differences  in  this  regard  between  related 
species }  To  these  questions  the  Roux-Weismann  theory  gives  a 
definite  and  intelligible  answer;  namely,  that  diffci^cntiation  sooner  or 
later  involves  a  specification  of  the  unclear  substance  zvJiicJi  differs  in 
degree  in  different  cases.  When  we  reflect  on  the  general  rdle  of  the 
nucleus  in  metabolism  and  its  significance  as  the  especial  seat  of  the 
formative  power,  we  may  well  hesitate  to  deny  that  this  part  of  Roux's 
conception  may  be  better  founded  than  his  critics  have  admitted. 
Nageli  insisted  that  the  idioplasm  must  undergo  a  progressive  trans- 
formation during  development,  and  many  subsequent  writers,  including 
such  acute  thinkers  as  Boveri  and  Nussbaum,  and  many  pathologists, 
have  recognized  the  necessity  for  such  an  assumption.  Boveri's  re- 
markable observations  on  the  nuclei  of  the  primordial  germ-cells  in 


322  THEORIES   OF  INHERITANCE  AND  DEVELOPMENT 

Ascaris  demonstrate  the  truth  of  this  view  in  a  particular  case  ;  for  here 
all  of  the  somatic  mtclei  lose  a  portion  of  their  chromatin,  and  07tly  the 
progenitors  of  the  germniiclci  rctaifi  the  entire  ancestral  heritage.  Boveri 
himself  has  in  a  measure  pointed  out  the  significance  of  his  discovery, 
insisting  that  the  specific  development  of  the  tissue-cells  is  condi- 
tioned by  specific  changes  in  the  chromatin  that  they  receive,^  though 
he  is  careful  not  to  commit  himself  to  any  definite  theory.  It  hardly 
seems  possible  to  doubt  that  in  Ascaris  the  limitation  of  the  somatic 
cells  in  respect  to  the  power  of  development  arises  through  a  loss  of 
particular  portions  of  the  chromatin.  One  cannot  avoid  the  thought 
that  further  and  more  specific  limitations  in  the  various  forms  of 
somatic  colls  may  arise  through  an  analogous  process,  and  that  we 
have  here  a  key  to  the  origin  of  nuclear  specification  zvithoiit  recourse 
to  the  theory  of  qualitative  division.  We  do  not  need  to  assume  that 
the  unused  chromatin  is  cast  out  bodily  ;  for  it  may  degenerate  and 
dissolve,  or  may  be  transformed  into  linin-substance  or  into  nucleoli. 
This  suggestion  is  made  only  as  a  tentative  hypothesis,  but  the 
phenomena  of  mitosis  seem  well  worthy  of  consideration  from  this 
point  of  view.  Its  application  to  the  facts  of  development  becomes 
clearer  when  we  consider  the  nature  of  the  nuclear  *'  control  "  of  the 
cell,  i.e.  the  action  of  the  nucleus  upon  the  cytoplasm.  Strasburger, 
following  in  a  measure  the  lines  laid  down  by  Nageli,  regards  the 
action  as  essentially  dynamic,  i.e.  as  a  propagation  of  molecular 
movements  from  nucleus  to  cytoplasm  in  a  manner  which  might  be 
compared  to  the  transmission  of  a  nervous  impulse.  When,  however, 
we  consider  the  role  of  the  nucleus  in  synthetic  metabolism,  and  the 
relation  between  this  process  and  the  morphological  formative  power, 
we  must  regard  the  question  in  another  light  ;  and  opinion  has  of 
late  strongly  tended  to  the  conclusion  that  nuclear  ''control"  can 
only  be  explained  as  the  result  of  active  exchanges  of  material 
between  nucleus  and  cytoplasm.  De  Vries,  followed  by  Hertwig, 
assumes  a  migration  of  pangens  from  nucleus  to  cytoplasm,  the 
character  of  the  cell  being  determined  by  the  nature  of  the  migrat- 
ing pangens,  and  these  being,  as  it  were,  selected  by  circumstances 
(position  of  the  cell,  etc.).  But,  as  already  pointed  out,  the  pangen 
hypothesis  should  be  held  quite  distinct  from  the  purely  physiologi- 
cal aspect  of  the  question,  and  may  be  temporarily  set  aside  ;  for 
specific  nuclear  substances  may  pass  from  the  nucleus  into  the 
cytoplasm  in  an  unorganized  form.  Sachs,  followed  by  Loeb,  has 
advanced  the  hypothesis  that  the  development  of  particular  organs 
is  determined  by  specific  "formative  substances"  which  incite  cor- 
responding forms  of  metabolic  activity,  growth,  and  differentiation. 

^  '91.  P-  433- 


THE   EXTERNAL    CONDITIONS   OF  DEVELOPMENT  323 

It  is  but  a  Step  from  this  to  the  very  interesting  suggestion  of 
Driesch  that  the  nucleus  is  a  storehouse  of  ferments  which  pass 
out  into  tj;ie  cytoplasm  and  there  set  up  specific  activities.  Under 
the  influence  of  these  ferments  the  cytoplasmic  organization  is  deter- 
mined at  every  step  of  the  development,  and  new  conditions  are 
established  for  the  ensuing  change.  This  view  is  put  forward  only 
tentatively  as  a  ''fiction"  or  working  hypothesis;  but  it  is  certainly 
full  of  suggestion.  Could  we  establish  the  fact  that  the  number  of 
ferments  or  formative  substances  in  the  nucleus  diminishes  with  the 
progress  of  differentiation,  we  should  have  a  comparatively  simple 
and  intelligible  explanation  of  the  specification  of  nuclei  and  the 
limitation  of  development.  The  power  of  regeneration  might  then 
be  conceived,  somewhat  as  in  the  Roux-Weismann  theory,  as  due  to 
a  retention  of  idioplasm  or  germ-plasm  —  i.e.  chromatin  —  in  a  less 
highly  modified  condition,  and  the  differences  between  the  various 
tissues  in  this  regard,  or  between  related  organisms,  would  find  a 
natural  explanation. 

Development  may  thus  be  conceived  as  a  progressive  transforma- 
tion of  the  egg-substance  primarily  incited  by  the  nucleus,  first  mani- 
festing itself  by  specific  changes  in  the  cytoplasm,  but  sooner  or  later 
involving  in  some  measure  the  nuclear  substance  itself.  This  process, 
which  one  is  tempted  to  compare  to  a  complicated  and  progressive 
form  of  crystallization,  begins  with  the  youngest  ovarian  ^g'g  and  pro- 
ceeds continuously  until  the  cycle  of  individual  life  has  run  its  course. 
Cell-division  is  an  accompaniment,  but  not  a  direct  cause  of  differen- 
tiation. The  cell  is  no  more  than  a  particular  area  of  the  germinal 
substance  comprising  a  certain  quantity  of  cytoplasm  and  a  mass  of 
idioplasm  in  its  nucleus.  Its  character  is  primarily  a  manifestation 
of  the  general  formative  energy  acting  at  a  particular  point  under 
given  conditions.  When  once  such  a  circumscribed  area  has  been 
established,  it  may,  however,  emancipate  itself  in  a  greater  or  less 
degree  from  the  remainder  of  the  mass,  and  acquire  a  specific  char- 
acter so  fixed  as  to  be  incapable  of  further  change  save  within  the 
limits  imposed  by  its  acquired  character. 


H.     The  External  Conditions  of  Development 

We  have  thus  far  considered  only  the  internal  conditions  of  devel- 
opment which  are  progressively  created  by  the  germ-cell  itself.  We 
must  now  briefly  glance  at  the  external  conditions  afforded  by  the 
environment  of  the  embryo.  That  development  is  conditioned  by 
the  external  environment  is  obvious.  But  we  have  only  recently 
come  to  realize  how  intimate  the  relation  is  ;  and  it  has  been  espc- 


324 


THEORIES   OF  INHERITANCE  AND   DEVELOPMENT 


cially  the  service  of  Loeb,  Herbst,  and  Driesch  to  show  how  essential 
a  part  is  played  by  the  environment  in  the  development  of  specific 
organic  forms.  The  limits  of  this  work  will  not  admit  of  any  adequate 
review  of  the  vast  array  of  known  facts  in  this  field,  for  which  the 
reader  is  referred  to  the  works  especially  of  Herbst.  I  shall  only 
consider  one  or  two  cases  which  may  serve  to  bring  out  the  general 
principle  that  they  involve.     Every  living  organism  at  every  stage 

of  its  existence  reacts  to  its  environ- 
ment by  physiological  and  morpho- 
logical changes.  The  developing 
embryo,  like  the  adult,  is  a  moving 
equilibrium  —  a  product  of  the  response 
of  the  inherited  organization  to  the 
external  stimuli  working  upon  it.  If 
these  stimuli  be  altered,  development 
is  altered.  This  is  beautifully  shown 
by  the  experiments  of  Herbst  and 
others  on  the  development  of  sea- 
urchins.  Pouchet  and  Chabry  showed 
that  if  the  embryos  of  these  animals 
be  made  to  develop  in  sea-water  con- 
taining no  lime-salts,  the  larva  fails  to 
develop  not  only  its  calcareous  skele- 
ton, but  also  its  ciliated  arms,  and  a 
larva  thus  results  that  resembles  in 
some  particulars  an  entirely  different 
specific  form ;  namely,  the  Tornaria 
larva  of  Balanoglossns.  This  result 
is  not  due  simply  to  the  lack  of  neces- 
sary material ;  for  Herbst  showed 
that  the  same  result  is  attained  if  a 
slight  excess  of  potassium  chloride  be 
added  to  sea-water  containing  the  nor- 
mal amount  of  lime  (Fig.  141).  In 
the  latter  case  the  specific  metabolism 
of  the  protoplasm  is  altered  by  a  particular  chemical  stimulus,  and  a 
new  form  results. 

The  changes  thus  caused  by  slight  chemical  alterations  in  the 
water  may  be  still  more  profound.  Herbst  ('92)  observed,  for  ex- 
ample, that  when  the  water  contains  a  very  small  percentage  of 
lithium  chloride,  the  blastula  of  sea-urchins  fails  to  invaginate  to 
form  a  typical  gastrula,  but  evaginates  to  form  an  hour-glass-shaped 
larva,  one  half  of  which  represents  the  archenteron,  the  other  half 
the  ectoblast.     Moreover,  a  much  larger  number  of  the  blastula-cells 


Fig.  141.  —  Normal  and  modified 
larvae  of  sea-urchins.     [HERBST.] 

A.  Normal  Pluteus  {Strongylocentro- 
ttis).  B.  Larva  {^Sphcer echinus)  at  the 
same  stage  as  the  foregoing,  developed 
in  sea-water  containing  a  slight  excess 
of  potassium  chloride. 


THE   EXTERNAL    CONDITIONS   OF  DEVELOPMENT  325 

undergo  the  differentiation  into  entoblast  than  in  the  normal  de- 
velopment, the  ectoblast  sometimes  becoming  greatly  reduced  and 
occasionally  disappearing  altogether,  so  that  the  entire  blastula  is 
differentiated  into  cells  having  the  histological  character  of  the  normal 
entoblast !     One  of  the  most  fundamental  of  embryonic  differentia- 


Fig.  142,  —  Regeneration  in  coelenterates  {A.  B.  from  LOEB;   C.  D.  from  BiCKFORD). 
, /.  Polyp   {Ceriantlms)   producing  new  tentacles   from   the  aboral  side  of  a  lateral  wound. 
D.  Hydroid  (  Tubularid)  generating  a  head  at  each  end  of  a  fragment  of  the  stem  suspended  in 
water.     C.  D.  Similar  generation  of  heads  at  both  ends  of  short  pieces  of  the  stem,  in  Tubularia. 

tions  is  thus   shown  to  be  intimately  conditioned  by  the    chemical 
environment. 

The  observations  of  botanists  on  the  production  of  roots  and  other 
structures  as  the  result  of  local  stimuli  are  familiar  to  all.  Loeb's 
interesting  experiments  on  hydroids  gave  a  similar  result  ('91).  It 
has  long  been  known  that  Tubularia,   like  many  other  hydroids,  has 


326  THEORIES   OF  INHERITANCE  AND  DEVELOPMENT 

the  power  to  regenerate  its  *'  head  "  —  i.e.  hypostome,  mouth,  and  ten- 
tacles—  after  decapitation.  Loeb  proved  that  in  this  case  the  power 
to  form  a  new  head  is  conditioned  by  the  environment.  For  if  a 
Tnbiilaria  stem  be  cut  off  at  both  ends  and  inserted  in  the  sand 
upside  down,  i.e.  with  the  oral  end  buried,  a  new  head  is  regen- 
erated at  the  free  (formerly  aboral)  end.  Moreover,  if  such  a  piece 
be  suspended  in  the  water  by  its  middle  point,  a  new  head  is  produced 
at  each  end  (Fig.  142);  while  if  both  ends  be  buried  in  the  sand, 
neither  end  regenerates.  •  This  proves  in  the  clearest  manner  that 
in  this  case  the  power  to  form  a  definite  complicated  structure  is 
called  forth  by  the  stimulus  of  the  external  environment. 

These  cases  must  suffice  for  our  purpose.  They  prove  incontesta- 
bly  that  nojinal  development  is  in  a  greater  or  less  degree  the  response 
of  the  developing  orgajtisin  to  normal  conditions  ;  and  they  show  that 
we  cannot  hope  to  solve  the  problems  of  development  without  reckon- 
ing with  these  conditions.  But  neither  can  we  regard  specific  forms 
of  development  as  directly  caused  by  the  external  conditions  ;  for  the 
^^<g  of  a  fish  and  that  of  a  polyp  develop,  side  by  side,  in  the  same 
drop  of  water,  under  identical  conditions,  each  into  its  predestined 
form.  Every  step  of  development  is  a  physiological  reaction,  involv- 
ing a  long  and  complex  chain  of  cause  and  effect  between  the  stimu- 
lus and  the  response.  The  character  of  the  response  is  determined 
not  by  the  stimulus,  but  by  the  inherited  organization.  While,  there- 
fore, the  study  of  the  external  conditions  is  essential  to  the  analysis 
of  embryological  phenomena,  it  serves  only  to  reveal  the  mode  of 
action  of  the  idioplasm  and  gives  but  a  dim  insight  into  its  ultimate 
nature. 

I.     Development,  Inheritance,  and  Metabolism 

In  bringing  the  foregoing  discussion  into  more  direct  relation  with 
the  general  theory  of  cell-action  we  may  recall  that  the  cell-nucleus 
appears  to  us  in  two  apparently  different  roles.  On  the  one  hand,  it 
is  a  primary  factor  in  morphological  synthesis  and  hence  in  inheri- 
tance, on  the  other  hand  an  organ  of  metabolism  especially  concerned 
with  the  constructive  process.  These  two  functions  we  may  with 
Claude  Bernard  regard  as  but  different  phases  of  one  process.  The 
building  of  a  definite  cell-product,  such  as  a  muscle-fibre,  a  nerve- 
process,  a  cilium,  a  pigment-granule,  a  zymogen-granule,  is  in  the  last 
analysis  the  result  of  a  specific  form  of  metabolic  activity,  as  we  may 
conclude  from  the  fact  that  such  products  have  not  only  a  definite 
physical  and  morphological  character,  but  also  a  definite  chemical 
character.  In  its  physiological  aspect,  therefore,  inheritance  is  the 
recurrence,  in  successive  generations,  of  like  forms  of  metabolism  ; 


PREFORMATION  AND   EPIGENESIS  32/ 

and  this  is  effected  through  the  transmission  from  generation  to  gen- 
eration of  a  specific  substance  or  idioplasm  which  we  have  seen 
reason  to  identify  with  chromatin.  This  remains  true  however  we 
may  conceive  the  morphological  nature  of  the  idioplasm — whether  as 
a  microcosm  of  invisible  germs  or  pangens,  as  conceived  by  De  Vries, 
Weismann,  and  Hertwig,  as  a  storehouse  of  specific  ferments  as 
Driesch  suggests,  or  as  complex  molecular  substance  grouped  in 
micellae  as  in  Nageli's  hypothesis.  It  is  true,  as  Verworn  insists, 
that  the  cytoplasm  is  essential  to  inheritance ;  for  without  a  specifi- 
cally organized  cytoplasm  the  nucleus  is  unable  to  set  up  specific 
forms  of  synthesis.  This  objection,  which  has  already  been  con- 
sidered from  different  points  of  view,  both  by  De  Vries  and  Driesch, 
disappears  as  soon  as  we  regard  the  egg-cytoplasm  as  itself  a  product 
of  the  7mclear  activity  ;  and  it  is  just  here  that  the  general  role  of  the 
nucleus  in  metabolism  is  of  such  vital  importance  to  the  theory  of 
inheritance.  If  the  nucleus  be  the  formative  centre  of  the  cell,  if 
nutritive  substances  be  elaborated  by  or  under  the  influence  of  the 
nucleus  while  they  are  built  into  the  living  fabric,  then  the  specific 
character  of  the  cytoplasm  is  determined  by  that  of  the  nucleus, 
and  the  contradiction  vanishes.  In  accepting  this  view  we  admit 
that  the  cytoplasm  of  the  ^g^  is,  in  a  measure,  the  substratum  of 
inheritance,  but  it  is  so  only  by  virtue  of  its  relation  to  the  nucleus, 
which  is,  so  to  speak,  the  ultimate  court  of  appeal.  The  nucleus 
cannot  operate  without  a  cytoplasmic  field  in  which  its  peculiar 
powers  may  come  into  play ;  but  this  field  is  created  and  moulded 
by  itself.  Both  are  necessary  to  development ;  the  nucleus  alone 
suffices  for  the  inheritance  of  specific  possibilities  of  development. 


J.    Preformation   and   Epigenesis.     The   Unknown    Factor    in 

Development 

We  have  now  arrived  at  the  furthest  outposts  of  cell-research  ;  and 
here  we  find  ourselves  confronted  with  the  same  unsolved  problems 
before  which  the  investigators  of  evolution  have  made  a  halt.  For 
we  must  now  inquire  what  is  the  guiding  principle  of  embryological 
development  that  correlates  its  complex  phenomena  and  directs  them 
to  a  definite  end.  However  we  conceive  the  special  mechanism  of 
development,  we  cannot  escape  the  conclusion  that  the  power  behind 
it  is  involved  in  the  structure  of  the  germ-plasm  inherited  from  fore- 
going generations.  What  is  the  nature  of  this  structure  and  how 
has  it  been  acquired  }  To  the  first  of  these  questions  we  have  as 
yet  no  certain  answer.  The  second  question  is  merely  the  general 
problem  of  evolution  stated  from  the  standpoint  of  the  cell-theory. 


328  THEORIES    OF  INHERITANCE  AND  DEVELOPMENT 

The  first  question  raises  once  more  the  old  puzzle  of  preformation 
or  epigenesis.  The  pangen  hypothesis  of  De  Vries  and  Weismann 
recognizes  the  fact  that  development  is  epigenetic  in  its  external 
features ;  but  like  Darwin's  hypothesis  of  pangenesis,  it  is  at  bottom 
a  theory  of  preformation,  and  Weismann  expresses  the  conviction 
that  an  epigenetic  development  is  an  impossibility.^  He  thus  ex- 
plicitly adopts  the  view,  long  since  suggested  by  Huxley,  that  *'the 
process  which  in  its  superficial  aspect  is  epigenesis  appears  in  es- 
sence to  be  evolution  in  the  modified  sense  adopted  in  Bonnet's  later 
writings ;  and  development  is  merely  the  expansion  of  a  potential 
organism  or  'original  preformation'  according  to  fixed  laws."^  Hert- 
wig  ('92,  2),  while  accepting  the  pangen  hypothesis,  endeavours  to 
take  a  middle  ground  between  preformation  and  epigenesis,  by 
assuming  that  the  pangens  (idioblasts)  represent  only  cell-cJiaracters, 
the  traits  of  the  multicellular  body  arising  epigenetically  by  permu- 
tations and  combinations  of  these  characters.  This  conception  cer- 
tainly tends  to  simplify  our  ideas  of  development  in  its  outward 
features,  but  it  does  not  explain  why  cells  of  different  characters 
should  be  combined  in  a  definite  manner,  and  hence  does  not  reach 
the  ultimate  problem  of  inheritance. 

What  lies  beyond  our  reach  at  present,  as  Driesch  has  very  ably 
urged,  is  to  explain  the  orderly  rhythm  of  development  —  the  co- 
ordinating power  that  guides  development  to  its  predestined  end. 
We  are  logically  compelled  to  refer  this  power  to  the  inherent 
organization  of  the  germ,  but  we  neither  know  nor  can  we  even 
conceive  what  this  organization  is.  The  theory  of  Roux  and  Weis- 
mann demands  for  the  orderly  distribution  of  the  elements  of  the 
germ-plasm  a  prearranged  system  of  forces  of  absolutely  incon- 
ceivable complexity.  Hertwig's  and  De  Vries's  theory,  though  ap- 
parently simpler,  makes  no  less  a  demand;  for  how  are  we  to 
conceive  the  power  which  guides  the  countless  hosts  of  migrating 
pangens  throughout  all  the  long  and  complex  events  of  development } 
The  same  difficulty  confronts  us  under  any  theory  we  can  frame.  If 
with  Herbert  Spencer  we  assume  the  germ-plasm  to  be  an  aggrega- 
tion of  like  units,  molecular  or  supra-molecular,  endowed  with  prede- 
termined polarities  which  lead  to  their  grouping  in  specific  forms, 
we  but  throw  the  problem  one  stage  further  back,  and,  as  Weismann 
himself  has  pointed  out,^  substitute  for  one  difficulty  another  of 
exactly  the  same  kind. 

The  truth  is  that  an  explanation  of  development  is  at  present 
beyond    our    reach.      The    controversy   between    preformation    and 

1  Germ-plasm,  p.  14. 

2  Evolution,  Science   and  Culture^  p.  296. 

^  Germinal  Selection,  January,  1896,  p.  284. 


PREFORMATION  AND  EPI GENESIS  329 

epigenesis  has  now  arrived  at  a  stage  where  it  has  little  meaning 
apart  from  the  general  problem  of  physical  causality.  What  we 
know  is  that  a  specific  kind  of  living  substance,  derived  from  the 
parent,  t^ids  to  run  through  a  specific  cycle  of  changes  during  which 
it  transforms  itself  into  a  body  like  that  of  which  it  formed  a  part  ; 
and  we  are  able  to  study  with  greater  or  less  precision  the  mechanism 
by  which  that  transformation  is  effected  and  the  conditions  under 
which  it  takes  place.  But  despite  all  our  theories  we  no  more  know 
how  the  properties  of  the  idioplasm  involve  the  properties  of  the 
adult  body  than  we  know  how  the  properties  of  hydrogen  and  oxygen 
involve  those  of  water.  So  long  as  the  chemist  and  physicist  are 
unable  to  solve  so  simple  a  problem  of  physical  causality  as  this, 
the  embryologist  may  well  be  content  to  reserve  his  judgment  on  a 
problem  a  hundredfold  more  complex. 

The  second  question,  regarding  the  historical  origin  of  the  idio- 
plasm, brings  us  to  the  side  of  the  evolutionists.  The  idioplasm  of 
every  species  has  been  derived,  as  we  must  believe,  by  the  modifica- 
tion of  a  pre-existing  idioplasm  through  variation,  and  the  survival 
of  the  fittest.  Whether  these  variations  first  arise  in  the  idioplasm 
of  the  germ-cells,  as  Weismann  maintains,  or  whether  they  may  arise 
in  the  body-cells  and  then  be  reflected  back  upon  the  idioplasm,  is 
a  question  on  which,  as  far  as  I  can  see,  the  study  of  the  cell  has 
not  thus  far  thrown  a  ray  of  light.  Whatever  position  we  take  on 
this  question,  the  same  difficulty  is  encountered;  namely,  the  origin 
of  that  co-ordinated  fitness,  that  power  of  active  adjustment  between 
internal  and  external  relations,  which,  as  so  many  eminent  biological 
thinkers  have  insisted,  overshadows  every  manifestation  of  life.  The 
nature  and  origin  of  this  power  is  the  fundamental  problem  of  biology. 
When,  after  removing  the  lens  of  the  eye  in  the  larval  salamander, 
we  see  it  restored  in  perfect  and  typical  form  by  regeneration  from 
the  posterior  layer  of  the  iris,^  we  behold  an  adaptive  response  to 
changed  conditions  of  which  the  organism  can  have  had  no  antece- 
dent experience  either  ontogenetic  or  phylogenetic,  and  one  of  so 
marvellous  a  character  that  we  are  made  to  realize,  as  by  a  flash  of 
light,  how  far  we  still  are  from  a  solution  of  this  problem.^  It  may 
be  true,  as  Schwann  himself  urged,  that  the  adaptive  power  of 
living  beings  differs  in  degree  only,  not  in  kind,  from  that  of  unor- 
ganized bodies.  It  is  true  that  we  may  trace  in  organic  nature  long 
and  finely  graduated  series  leading  upward  from  the  lower  to  the 
higher  forms,  and  we  must  believe  that  the  wonderful  adaptive  mani- 
festations of  the  more  complex  forms  have  been  derived  from  simpler 
conditions  through  the  progressive  operation  of  natural  causes.     Rut 

1  See  Wolff,  '95,  and  Miiller,  '96. 

2  See  Wolff,  '94,  for  an  admirably  clear  and  forcible  discussion  of  this  case. 


330  THEORIES   OF  INHERITANCE  AND  DEVELOPMENT 

when  all  these  admissions  are  made,  and  when  the  conserving 
action  of  natural  selection  is  in  the  fullest  degree  recognized,  we  can- 
not close  our  eyes  to  two  facts  :  first,  that  we  are  utterly  ignorant  of 
the  manner  in  which  the  idioplasm  of  the  germ-cell  can  so  respond 
to  the  play  of  physical  forces  upon  it  as  to  call  forth  an  adaptive 
variation ;  and  second,  that  the  study  of  the  cell  has  on  the  whole 
seemed  to  widen  rather  than  to  narrow  the  enormous  gap  that  sepa- 
rates even  the  lowest  forms  of  life  from  the  inorganic  world. 

I  am  well  aware  that  to  many  such  a  conclusion  may  appear  reac- 
tionary or  even  to  involve  a  renunciation  of  what  has  been  regarded 
as  the  ultimate  aim  of  biology.  In  reply  to  such  a  criticism  I  can 
only  express  my  conviction  that  the  magnitude  of  the  problem  of 
development,  whether  ontogenetic  or  phylogenetic,  has  been  under- 
estimated ;  and  that  the  progress  of  science  is  retarded  rather  than 
advanced  by  a  premature  attack  upon  its  ultimate  problems.  Yet 
the  splendid  achievements  of  cell-research  in  the  past  twenty  years 
stand  as  the  promise  of  its  possibilities  for  the  future,  and  we  need 
set  no  limit  to  its  advance.  To  Schleiden  and  Schwann  the  present 
standpoint  of  the  cell-theory  might  well  have  seemed  unattainable. 
We  cannot  foretell  its  future  triumphs,  nor  can  we  repress  the  hope 
that  step  by  step  the  way  may  yet  be  opened  to  an  understanding  of 
inheritance  and  development. 


LITERATURE.     IX 

Boveri,  Th.  —  Ein  geschlechtlich  erzeugter  Organismus  ohne  mlitterliche  Eigen- 
schaften:  Sitz.-Ber.  d.  Ges.f.  Morph.  tmd  Phys.  in  Milnchen,  V.  1889.  See 
also  Arch .  f.  Entwm .     1895. 

Brooks,  W.  K.  —  The  Law  of  Heredity.     Baltimore,  1883. 

Driesch,  H.  — Analytische  Theorie  der  organischen  Entwicklung.     Leipzig,  1894. 

Herbst,  C.  —  Uber  die  Bedeutung  der  Reizphysiologie  fUr  die  kausale  Auffassung 
von  Vorgangen  in  der  tierischen  Ontogenese:  BioL  Centralb.,  XIV.,  XV. 
1894-95. 

Hertwig,  0.  —  Altere  und  neuere  Entwicklungs-theorieen.     Berliti,  1892. 

Id.  — Urmund  und  Spina  Bifida:  Arch.  mik.  Anat.,  XXXIX.     1892. 

Id.  —  Uber  den  Werth  der  ersten  Furchungszellen  fiir  die  Organbildung  des 
Embryo  :  Arch.  mik.  Anat.,  XLII.     1893. 

Id. — Zeit  und  Streitfragen  der  Biologic.     Berlin.,  1894. 

His,  W.  —  Unsere  Korperform  und  das  physiologische  Problem  ihrer  Entstehung. 
Leipzig,  1874. 

Loeb,  J.  —  Untersuchungen  zur  physiologischen  Morphologie  :  I.  Heteromorphosis. 
Wiirzburg,  \%c^\ .     II.  Organbildung  und  Wachsthum.     IViirzdurg,  iSg2. 

Id.  —  Some  Facts  and  Principles  of  Physiological  Morphology:  Wood's  Holl  Biol. 
Lectures.     1893. 

Nageli,  C.  —  Mechanisch-physiologische  Theorie  der  Abstammungslehre.  Miln- 
chen u.  Leipzig,  1884. 

Roux,  W.  —  Tiber  die  Bedeutung  der  Kernteilungsfiguren.     Leipzig.  1883. 


PREFORMATION  AND   EPICENE  SIS  33  I 

Roux,  W.  —  iJber  das  kiinstliche  Hervorbringen  halber  Embryonen  durch  Zerstorung 
einer  der  beiden  ersten  Furchungskugeln,  etc. :    Virchow's  Archiv^  114.     1888. 

Sachs,  J.  —  Stoff  und  Form  der  Pflanzenorgane  :  Ges.  Abhandlungen^  II.     1893. 

Weismann,  A.  —  Essays  upon  Heredity,  First  Series.     Oxford^  1891. 

Id.  —  Essays  upon  Heredity,  Second  Series.     Oxford,  1892. 

Id.  —  Aussere  Einflusse  als  Entwicklungsreize.     Jena,  1894. 

Whitman,  C.  0.  —  Evolution  and  Epigenesis  :   lVood''s  Holl  Biol.  Lectures.     1894. 

Wilson,  Edm.  B.  —  On  Cleavage  and  Mosaic-work:  Arch,  fur  Entwicklungsm., 
III.  I.     1896. 


GLOSSARY 


[Obsolete  terms  are  enclosed  in  brackets.    The  name  and  date  refer  to  the  first  use  of  the  word  ; 
subsequent  changes  of  meaning  are  indicated  in  the  definition.] 

Achro'matin    (see    Chromatin),  the    non-staining   substance  of  the   nucleus,  as 

opposed  to  chromatin ;  comprising  the  ground-substance  and  the  linin-network. 

(Flemming.  1880.) 
[Akaryo'ta]  (see  Karyota),  non-nucleated  cells.     (Flemming,  1882.) 
Ale'cithal  (d-priv. ;  XeKt^o?,  the  yolk  of  an  egg),  having  little  or  no  yolk  (applied 

to  eggs).     (Balfour,  1880.) 
Amito'sis   (see  Mitosis),  direct  or  amitotic  nuclear   division;    mass-division  of 

the  nuclear  substance  without  the  formation  of  chromosomes  and  amphiaster. 

(Flemming.  1882.) 
Ani'phiaster  (d/>t^t',  on  both  sides;  daTrjpj  a  star),  the  achromatic  figure  formed 

in  mitotic  cell-division,  consisting  of  two  asters  connected  bv  a  spindle.     (FoL, 

1877.) 

Amphipy 'renin  (see  Pyrenin),  the  substance  of  the  nuclear  membrane. 
(ScHWARZ,  1887.) 

Amyloplasts  (afxvXov,  starch;  TrAao-ro?,  TrXdo-cretv,  form),  the  colourless  starch- 
forming  plastids  of  plant-cells.     (Errara,  1882.) 

An'aphase  (avd,  back  or  again),  the  later  period  of  mitosis  during  the  divergence 
of  the  daughter-chromosomes.     (Strasburger,  1884.) 

Aniso'tropy  (see  Isotropy),  having  a  predetermined  axis  or  axes  (as  applied  to 
the  egg).     (Pfluger,  1883.) 

Anther ozo'id,  the  same  as  Spermatozoid. 

Anti'podal  cone,  the  cone  of  astral  rays  opposite  to  the  spindle-fibres.  (Van 
Beneden,  1883.) 

Archiani'phiaster  (dpxi-  =  fii"st,  +  amphiaster),  the  amphiaster  by  which  the  first 
or  second  polar  body  is  formed.     (Whitman,  1878.) 

Ar'choplasnia  or  Archoplasm  (dpxtjov,  a  ruler),  the  substance  from  which  the 
attraction-sphere,  the  a:stral  rays  and  the  spindle-fibres  are  developed,  and  of 
which  they  consist.     (Boveri,   1888.) 

As'ter  (da-T^p,  a  star),  i.  The  star-shaped  structure  surrounding  the  centrosome. 
(FoL,  1877.)  [2.  The  star-shaped  group  of  chromosomes  during  mitosis  (see 
Karyaster).      (FleMxMING,  1892.)] 

[As'trocoele]  (daTyp,  a  star ;  koiAo?,  hollow),  a  term  somewhat  vaguely  applied  to 
the  space  in  which  the  centrosome  lies.     (Fol,  1891.) 

As'trosphere  (see  Centrosphere) .  i.  The  central  mass  of  the  aster,  exclusive 
of  the  rays,  in  which  the  centrosome  lies.  Equivalent  to  the  "attraction- 
sphere"  of  Van  Beneden.  (Fol,  1891  ;  Strasburger.  1892.)  2.  The  entire 
aster  exclusive  of  the  centrosome.  Equivalent  to  the  "astral  sphere"  of 
Mark.     (Boveri,  1895.) 

333 


334  GLOSSARY 

Attraction-sphere  (see  Centrosphere),  the  central  mass  of  the  aster  from  which 
the  ravs  proceed.  Also  the  mass  of  "  archoplasm/'  derived  from  the  aster,  by 
which  the  centrosome  is  surrounded  in  the  resting  cell.     (Van  Beneden,  1883.) 

[Au'toblast]  (ttVTo?,  self),  applied  by  Altmann  to  bacteria  and  other  minute  organ- 
isms, conceived  as  independent  solitary  "  bioblasts."     (1890.) 

Axial  filament,  the  central  filament,  probably  contractile,  of  the  spermatozobn- 
fiagellum.     (Elmer,  1874.) 

Basichro 'matin  (see  Chromatin),  the  same  as  chromatin  in  the  usual  sense. 
That  portion  of  the  nuclear  network  stained  by  basic  aniline  dyes.  (Heidenhain, 
1894.) 

Bi'oblast  (/8tos,  life ;  /?A.acrro?,  a  germ),  the  hypothetical  ultimate  supra-molecular 
vital  unit.  Equivalent  to  plasoine^  etc.  First  used  by  Beale.  Afterwards  identi- 
fied by  Altmann  as  the  "  granulum." 

Bi'ogen   (/t?to?,  life:  -ycvrj?,  producing),  equivalent  \.o  plasonie,  etc.     (Verworx, 

1895.) 
Bi'ophores  (^to?,  life ;  -<f>6po<;,  bearing),  the  ultimate  supra-molecular  vital  units. 

Equivalent  to  the  pangens  of  De  Vries,  the  plasomes  of  Wiesner,  etc.   (Weismann, 

1893.) 
Biva'lent,  applied  to  chromatin-rods  representing  two  chromosomes  joined  end  to 

end.     (Hacker,  1892.) 
Cell-plate  (see  Mid-body),  the  equatorial  thickening    of  the  spindle-fibres  from 

which  the  partition-wall  arises  during  the  division  of  plant-cells.      (Strasbur- 

GER,  1875.) 
Cell-sap,  the  more  liquid  ground-substance  of  the  nucleus.       [Kolliker,    1865  ; 

more  precisely  defined  by  R.  Hertwig,  1876.] 
Central  spindle,  the  primary  spindle  by  which  the  centrosomes  are  connected,  as 

opposed  to  the  contractile  mantle-fibres  surrounding  it.     (Hermann,  1891.) 
Centriole,  a  term  applied  by  Boveri  to  a  minute  body  or  bodies  ("  Central-korn  ") 

within  the  centrosome.     In  some  cases  not  to  be  distinguished  from  the  centro- 
some.    (Boveri,  1895.) 
Centrodes'mus  (Kevrpov,  centre ;  Sea/Aog,  a  band),  the  primary  connection  between 

the  centrosomes,  forming  the  beginning  of  the  central  spindle.     (Heidenhain, 

1894.) 
Centrole'cithal  (KcVrpov,  centre ;  AcKt^o?,  yolk),  that  type  of  ovum  in  which  the 

deutoplasm  is  mainly  accumulated  in  the  centre.     (Balfour,  1880.) 
Cen'trosome  (Kevrpov,  centre ;  o-co/xa,  body),  a  cell-organ  generally  regarded  as  the 

active  centre  of  cell-division  and  in  this  sense  as  the  dynamic  centre  of  the  cell. 

Under  its  infiuence   arise  the  asters   and  spindle  (amphiaster)    of  the  mitotic 

figure.     (Boveri,  1888.) 
Cen'trosphere,  used  in  this  work  as  equivalent   to   the  "  astrosphere "  of  Stras- 

burger ;  the  central  mass  of  the  aster  from  which  the  rays  proceed  and  within 

which   lies   the   centrosome.      The   attraction-sphere.      [Strasburger,    1892; 

applied  by  him  to  the  "  astrosphere  "  and  centrosome  taken  together.] 
Chloroplas'tids  (;^A(jopo9,  green  ;  TrAacrTo?,  form),  the  green  plastids  or  chlorophyll- 
bodies  of  plant  and  animal  cells.     (Schimper,  1883.) 
Chro'matin  (xpiop-a,  colour),  the  deeply  staining  substance  of  the  nuclear  network 

and  of  the  chromosomes,  consisting  of  nuclein  or  nucleic  acid.     (Flemming, 

1880.) 
Chro'matophore   (xpo)fJiOL,  colour;  -ff>6po<i,  bearing),  a  general  term  applied  to  the 

colored  plastids  of  plant  and  animal  cells,  including  chloroplastids  and  chromo- 

plastids.     (Schaarschmidt,  1880;  Schmitz,  1882.) 
Chro'matoplasm  (p^pui/xa,  colour;  TrAacr/xa,  anything  formed  or  moulded),  the  sub- 
stance of  the  chromatoplasts  and  other  plastids.     (Strasburger,  1882.) 


GLOSSAKV  335 

Cliro'momere  (xpoifia,  colour;  fxipo^,  a  part),  the  individual  chromatin-granules  of 
which  the  chromosomes  are  made  up.  Identified  by  Weismann  as  the  *' id." 
(FoL,  1 891.) 

Chromoplas'tids  {^puifxa,  colour;  TrAacTTos,  form),  the  coloured  plastids  or  pigment- 
bodies  other  than  the  chloroplasts,  in  plant-cells.     (Schimper,  1883.) 

Chro'mosoraes  {^piofxa,  colour  ;  crw/Lta,  body),  the  deeply  staining  bodies  into  which 
the  chromatic  nuclear  network  resolves  itself  during  mitotic  cell-division.  (Wal- 
DEYER,  1888.) 

Cleavage-nucleus,  the  nucleus  of  the  fertilized  egg,  resulting  from  the  union  of 
egg-nucleus  and  sperm-nucleus.     (O.  Hertwig,  1875.) 

Cortical  zone,  the  outer  zone  of  the  centrosphere.     (Van  Beneden,  1887.) 

Cyano'philous  {^Kvavo^,  blue;  cfuXelv,  to  love),  having  an  especial  affinity  for  blue 
or  green  dyes.     (Auerbach.) 

Cy 'taster  {KVTO<i,  hollow  (a  cell);  daryp,  star),  the  same  as  Aster,  i.  See  Kary- 
aster.     (Flemming,  1882.) 

[Cy'toblast]  (kwos,  hollow  (a  cell);  jSXaa-To^,  germ),  i.  The  cell-nucleus. 
(SCHLEIDEN,  1838.)  2.  One  of  the  hypothetical  ultimate  vital  units  (bioblasts  or 
''granula")  of  which  the  cell  is  built  up.  (Altmann,  1890.)  3.  A  naked  cell 
or  "protoblast.''     (Kolliker.) 

[Cytoblaste'ma]  (see  Cytoblast),  the  formative  material  from  which  cells  were 
supposed  to  arise  by  "free  cell-formation."     (Schleiden,  1838.) 

Cytochyle'ma  (kvto<;,  hollow  (a  cell)  ;  x^^®?'  juice),  the  ground-substance  of  the 
cytoplasm  as  opposed  to  that  of  the  nucleus.     (Strasburger,  1882.) 

Cy'tode  (kvtosj  hollow  (a  cell) ;  eT8o<ij  form),  a  non-nucleated  cell.     (Hackel,  1866.) 

Cytodie'resis  (kvto<;,  hollow  (a  cell)  ;  Statpecris,  division),  the  same  as  Mitosis. 
(Carnoy,  1885.) 

Cytohy'aloplasma  (kvto^,  hollow  (a  cell)  ;  vaXo?,  glass  ;  TrXaa-jxa,  anything  formed), 
the  substance  of  the  cytoreticulum  in  which  are  embedded  the  microsomes ; 
opposed  to  nucleohyaloplasma.     (Strasburger,  1882.) 

Cy'tolymph  {kvto<;,  hollow  (a  cell)  ;  lynipha,  clear  water),  the  cytoplasmic  ground- 
substance.     (Hackel,  1891.) 

Cytomi'crosomes  (see  Microsome),  microsomes  of  the  cytoplasm;  opposed 
to  nucleomicrosomes.     (Strasburger,  1882.) 

Cytomi'tome  {Kvro<i^  hollow  (a  cell)  ;  /jiLTOiiJui,  from  /xtVo?,  thread),  the  cytoplasmic 
as  opposed  to  the  nuclear  thread- work.     (Flemming,  1882.) 

Cytoreticulum,  the  same  as  Cytomitome.     (Strasburger,  1882.) 

Cy'tosome  (kvto^,  hollow  (a  cell) ;  (juifxa^  body,)  the  cell-body  or  cytoplasmic 
mass  as  opposed  to  the  nucleus.     (Hackel,  1891.) 

Der'matoplasm  (Sep/xa,  skin),  the  living  protoplasm  asserted  to  form  a  part  of  the 
cell-membrane  in  plants.     (Wiesner,  1886.) 

Der'matosomes  (Sep/xa,  skin ;  crw/xa,  body),  the  plasomes  which  form  the  cell-mem- 
brane.    (Wiesner,  1886,) 

Determinant,  a  hypothetical  unit  formed  as  an  aggregation  of  biophores,  determin- 
ing the  development  of  a  single  cell  or  independently  variable  group  of  cells. 
(Weismann,  1891.) 

[Deuthy'alosome]  (8f.vT(epo<;),  second  ;  see  Hyalosome),  the  nucleus  remaining 
in  the  egg  after  formation  of  the  first  polar  body.     (Van  Beneden,  1883.) 

Deu'toplasm  (8eL'r(epos),  second ;  irXda/xaj  anything  formed),  yolk,  lifeless  food- 
matters  deposited  in  the  cytoplasm  of  the  egg;  opposed  to  "protoplasm."  (Van 
Beneden,  1870.) 

Directive  bodies,  the  polar  bodies.     (Fr.  Muller,  1848.) 

Directive  sphere,  the  attraction-sphere.     (Guignard,  1891.) 

Disperniy,  the  entrance  of  two  spermatozoa  into  the  egg. 


^7,0  GLOSSAKY 

Dispi'reme  (see  Spireme),  that  stage  of  mitosis  in  which  each  daughter-nucleus 

has  given  rise  to  a  spireme.     (Flemming,  1882.) 
Dy 'aster  (Sua?,  two;  see  Aster,  2),  the  double  group  of  chromosomes  during  the 

anaphases  of  cell-division.     (Flemming,  1882.) 
Egg-nucleus,  the  nucleus  of  the  egg  after  formation  of  the  polar  bodies  and  before 

its  union  with  the  sperm-nucleus.    Equivalent  to  the  ''female  pronucleus"  of  Van 

Ben- EDEN.     (O.  Hektwig,  1875.) 
Enchyle'ma  (tV,  in;   x^^^os?  juice),     i.  The   more   fluid  portion   of  protoplasm, 

consisting   of  "hyaloplasma."     (Hanstein,    1882.)      2.  The   ground-substance 

(cytolymph)  of  cytoplasm  as  opposed  to  the  reticulum.     (Carnov^,  1883.) 
Ener'gid,  the  cell-nucleus  together  with   the  cytoplasm  lying  within  its  sphere  of 

influence.     (Sachs,  1892.) 
Equatorial  plate,  the  group  of  chromosomes  lying  at  the  equator  of  the  spindle 

during  mitosis.     (Van  Beneden,  1875.) 
Erythro'philous   (ipvOpog,  red ;   ^lAeti/,   to  love),  having  an  especial  affinity  for 

red  dyes.     (Auerbach.) 
Ga'mete  (yajucrr;,  wife  ;  ya/xerTys,  husband),  one  of  two  conjugating  cells.     Usually 

applied  to  the  unicellular  forms. 
Geni'mule   (see  Pangeii),  one  of  the  ultimate  supra-molecular  germs  of  the  cell 

assumed  by  Darwin.     (Darwin,  1868.) 
[Ge'noblasts]  (yeVos,  sex ;  ^Aao-ro?,  germ),  a  term  applied  by  Minot  to  the  mature 

germ-cells.     The  female  genoblast  (egg,  or  "  thelyblast ")  unites  with  the  male 

(spermatozoon  or  '•  arsenoblast ")  to  form  an  hermaphrodite  or  indiff'erent  cell. 

(MixoT,  1877.) 
Germinal  spot,  the  nucleolus  of  the  germinal  vesicle.     (Wagner,  1836.) 
Germinal  vesicle,  the   nucleus  of  the  egg  before  formation  of  the  polar  bodies. 

(Purkinje,  1825.) 
Germ-plasm,  the  same  as  idioplasm.     (Weismann.) 
Heterole'cithal    (erepo^,   diff"erent ;    AcKt^os,  yolk),  having   unequally   distributed 

deutoplasm  (includes  telolecithal  and  centrolecithal).     (Mark,  1892.) 
Heterotyp'ical  mitosis    (erepo's,  different;   see  Mitosis),  that  mode  of  mitotic 

division  in  which  the  daughter-chromosomes  remain  united  bv  their  ends  to  form 

rings.     (Flemming,  1887.) 
[Holoschi'sis]  (oAos,  whole;  o-^t'^eti/,  to  split),  direct  nuclear  division.     Amitosis. 

(Flemming,  1882.) 
Homole'cithal   (6/xo?,  the  same,  uniform;  AeKi(9o?,  yolk),  equivalent  to  alecithal. 

Having  little  deutoplasm,  equally  distributed,  or  none.     (Mark,  1892.) 
Homoeotyp'ical  mitosis  (^/xoio?,  like;  see  Mitosis),  a  form  of  mitosis  occurring 

in  the  spermatocytes  of  the  salamander,  differing  from  the  usual  type  only  in  the 

shortness  of  the  chromosomes  and  the  irregular  arrangement  of  the  daughter- 
chromosomes.     (Flemming,  1887.) 
Hy'aloplasma  (va\o<;,  glass;    TrXdafxa,  anything  formed),     i.    The    ground-sub- 
stance of  the  cell  as  distinguished  from  the  granules  or  microsomes.     [Hanstein, 

1880.]     2.  The  ground-substance  as  distinguished  from  the  reticulum  or  "spon- 
gioplasm."     (Leydig,  1885.)     3.  The  exoplasm  or  peripheral  protoplasmic  zone 
in  plant-cells.     (Pfeffer.) 
Hy'alosomes  (voAo?,  glass;  o-w/xa,  body),  nucleolar-like  bodies  but  slightly  stained 

by  either  nuclear  or  plasma  stains.     (Lukjanow,  1888.) 
[Hy'groplasma]  (vypo?,  wet ;  TrAacr^a,  something  formed),  the  more  liquid  part 

of  protoplasm  as  opposed  to  the  firmer  stereoplasm.     (Nageli,  1884.) 
Id,  the  hypothetical  structural   unit    resulting  from  the    successive  aggregation  of 
biophores  and  determinants.     Identified  by  Weismann  as  the   chromomere,  or 
chromatin-granule.     (Weismann,  1891.) 


GLOSSARY 


337 


Idant,  the  hypothetical  unit  resulting  from  the  successive  aggregation  of  biophores, 

determinants,  and  ids.     Identified  by  Weismann  as  the  chromosome.     (Weis- 

MANN,  1891.) 
Id'ioblasts  (t8io?,  one's  own,  ^Xaaroq,  germ),  the  hypothetical  ultimate  units  of  the 

cell;  the^ame  as  biophores.     (O.  Hertwig,  1893.) 
Idioplasm  (tStos,  one's  own;  TrAafr/xa,  a  thing  formed),  equivalent  to  the  germ- 
plasm  of  Weismann.     The  substance,  now  generally  identified  with   chromatin, 

which  by  its  inherent  organization  involves  the  characteristics  of  the  species. 

The  physical  basis  of  inheritance.     (Nageli,  1884.) 
Id'iosome    (t6to?,    one's    own;  cra>/xa,  body),  the  same  as    idioblast   or   plasome. 

(Whitman,  1893.) 
Interfilar   substance,    the   ground-substance    of   protoplasm   as  opposed   to    the 

thread- work.     (Flemming,  1882.) 
Interzonal  fibres  ("Filaments  reunissants"    of   Van    Beneden.     " Verbindungs- 

fasern  "  of  Flemming  and  others) .     Those  spindle-fibres  that  stretch  between 

the  two  groups  of  daughter-chromosomes  during  the  anaphase.     Equivalent  in 

some  cases  to  the  central  spindle.     (Mark,  1881.) 
Iso'tropy  (tVos,  equal;  Tpo-n-rj,  a  turning),  the  absence  of  predetermined  axes  (as 

applied  to  the  egg).     (Pfluger,  1883.) 
[Ka'ryaster]  {Kapvov,  nut,  nucleus  ;  see  Aster,  2),  the  star-shaped  group  of  chromo- 
somes in  mitosis.     Opposed  to  cytaster.     (Flemming,  1882.) 
Karyenchy'ma  {Kapvov,  nut,  nucleus  ;  ei/,  in ;  x^A"-^^'   juice),  the  ''  nuclear   sap." 

(Fleiviming,  1882.) 
Karyokine'sis    {Kapvov,  nut,  nucleus  ;    KLvr)aL<;,  change,  movement),  the  same  as 

mitosis.     (Schleicher,  1878.) 
[Karyoly'ma] ,  the  "■  karyolytic  "  (mitotic)  figure.     (Auerbach,  1876.) 
Ka'ryolymph.     The  nuclear  sap.     (Hackel,  1891.) 
[Karyo'lysis]  (Kapvov,  nut,  nucleus ;  Xvais,  dissolution),  the  supposed  dissolution 

of  the  nucleus  during  cell-division.     (Auerbach,  1874.) 
[Karyoly'tic   figure]    (see   Karyolysis),   a   term    applied   by   Auerbach   to   the 

mitotic  figure  in  living  cells.     Believed  by  him  to  result  from  the  dissolution  of 

the  nucleus.     (Auerbach,  1874.) 
Karyomi'crosonie  (see  Microsome),  the  same  as  nucleo-microsome. 
Ka'ryomite,  the  same  as  chromosome  [?  Schiefferdecker]. 
Karyonii'tome  (Kapvov,  nut,  nucleus  ;  /xira)/x,a,  from  /uttro?,  a  thread),  the  nuclear 

as  opposed  to  the  cytoplasmic  thread-work.     (Flemming,  1882.) 
Karyomito'sis     (Kapvov,    nut,    nucleus;     see    Mitosis),     mitosis.       (Flemming, 

1882.) 
Ka'ryon  (Kapvov,  nut,  nucleus),  the  cell-nucleus.      (Hackel,  1891.) 
Ka'ryoplasm  (Kapi^ov,  nut,  nucleus  ;    7rAa(r/xa,  a  thing  formed),  nucleoplasm.     The 

nuclear  as  opposed  to  the  cytoplasmic  substance.     (Flemming,  1882.) 
Ka'ryosome  (Kapvov,  nut,  nucleus;  crw/xa,  body),     i.  Nucleoli  of  the  "net-knot" 

type,  staining  with  nuclear  dyes,  as  opposed  to  plasmosomes  or  true  nucleoli. 

(Ogata,  1883.)     2.  The  same  as  chromosome.     (Platner,  1886.)     3.  Caryo- 

some.     The  cell-nucleus.     (Watase,  1894.) 
[Karyo'ta]  (Kapvov,  nut,  nucleus),  nucleated  cells.     (Flemming,  1882.) 
Karyothe'ca    (Kapvov,   nut,    nucleus;    ^17/cr;,    case,    box),    the    nuclear    membrane. 

(Hackel,  1891.) 
Ki'noplasm  (Ktvtiv,  to  move  ;  TrAaa/xa,  a  thing  formed),  equivalent  to  archoplasm ; 

opposed    by   Strasburger    to    the   "  trophoplasm "   or  nutritive  plasm.     (Stras- 

BURGER,  1892.) 
[Lanthanin]     (XavOdveiv,    to    conceal),    equivalent    to    ox3^chromatin.      (Heiden- 

hain.  1892.) 
z 


338 


GLOSSARY 


Leucoplas'tida  (Xcvko?,  white ;  TrXacrros,  form),  the  colourless  plastids  of  plant- 
cells  from  which  arise  the  starch-formers  (amyloplastids),  chloroplastids,  and 
chromoplastids.     (Schimper,  1883.) 

Li'nin  (linum,  a  linen  thread),  the  substance  of  the  "achromatic"  nuclear 
reticulum.     (Schwarz,  1887.) 

Maturation,  the  final  stages  in  the  development  of  the  germ-cells.  More  spe- 
cifically, the  processes  by  which  the  reduction  of  the  number  of  chromosomes 
is  effected. 

Metakine'sis  (see  Metaphase)  (/xera,  beyond  {i.e.  further)  ;  Ktvvrjcn^,  movement), 
the  middle  stage  of  mitosis,  when  the  chromosomes  are  grouped  in  the  equa- 
torial plate.     (Flemmlng,  1882.) 

Metanu'cleus,  a  term  applied  to  the  egg-nucleus  after  its  extrusion  from  the 
germinal  vesicle.     (Hacker,  1892.) 

Met'aphase,  the  middle  stage  of  mitosis  during  which  occurs  the  splitting  of  the 
chromosomes  in  the  equatorial  plate.     (Strasburger,  1884.) 

Met'aplasm  (fxerd,  after,  beyond;  TrXda-fm,  a  thing  formed),  a  term  collectively 
applied  to  the  lifeless  inclusions  (deutoplasm,  starch,  etc.)  in  protoplasm  as  op- 
posed to  the  living  substance.     (Hanstein,  1880.) 

Micella,  one  of  the  ultimate  supra-molecular  units  of  the  cell.     (Nageli,  1884.) 

Microcen'trum,  the  dynamic  centre  of  the  cell,  consisting  of  one  or  more  centro- 
somes.     (Heidenhain,  1894.) 

Mi'cropyle  (/uiKpos,  small;  7rvX.r},  orifice),  the  aperture  in  the  egg-membrane 
through  which  the  spermatozoon  enters.  [First  applied  by  Turpin,  in  1806, 
to  the  opening  through  which  the  pollen-tube  enters  the  ovule,  t.  Robert 
Brown.] 

Mi'crosome  (/xiKpoq,  small ;  crtu/Aa,  body),  the  granules  as  opposed  to  the  ground- 
substance  of  protoplasm.     (Hanstein,  1880.) 

Middle-piece,  that  portion  of  the  spermatozoon  lying  behind  the  nucleus  at  the 
base  of  the  flagellum.     (Schweigger-Seidel,  1865.) 

Mid-body  ("Zwischenkbrper"),  a  body  or  group  of  granules,  probably  comparable 
with  the  cell-plate  in  plants,  formed  in  the  equatorial  region  of  the  spindle  during 
the  anaphases  of  mitosis.     (Flemming,  1890.) 

Mi'tome  (^trw/xa,  from  /u,tro5,  a  thread),  the  reticulum  or  thread- work  as  opposed 
to  the  ground-substance  of  protoplasm.     (Flemming,  1882.) 

[Mitoschi'sis]  (fiLTosy  thread;  crxt'^eii/,  to  split),  indirect  nuclear  division;  mito- 
sis.    (Flemming,  1882.) 

Mito'sis  (fiLTo^;,  a  thread),  indirect  nuclear  division  typically  involving:  <?,  the 
formation  of  an  amphiaster;  d,  conversion  of  the  chromatin  into  a  thread 
(spireme)  ;  c,  segmentation  of  the  thread  into  chromosomes ;  d,  splitting  of  the 
chromosomes.     (Flemming,  1882.) 

Mi'tosome  {jxito^,  a  thread ;  aiofm,  body),  a  body  derived  from  the  spindle-fibres 
of  the  secondary  spermatocytes,  giving  rise,  according  to  Platner,  to  the  mid- 
dle-piece and  the  tail-envelope  of  the  spermatozoon.  Equivalent  to  the  Neben- 
kern  of  La  Valette  St.  George.     (Platner,  1889.) 

Nebenkern  (Paranucleus),  a  name  originally  applied  by  BUtschli  (1871)  to  an 
extranuclear  body  in  the  spermatid  ;  afterwards  shown  by  La  Valette  St.  George 
and  Platner  to  arise  from  the  spindle-fibres  of  the  secondary  spermatocyte. 
Since  applied  to  many  forms  of  cytoplasmic  bodies  (yolk-nucleus,  etc.)  of  the 
most  diverse  nature. 

Nuclear  plate,  i.  The  equatorial  plate.  (Strasburger,  1875.)  2.  The  parti- 
tion-wall whicli  sometimes  divides  the  nucleus  in  amitosis. 

Nucleic  acid,  a  complex  organic  acid,  rich  in  phosphorus,  and  an  essential 
constituent  of  chromatin. 


GLOSSARY 


339 


Nuclein,  the  chemical  basis  of  chromatin;  a  compound  of  nucleic  acid  and 
albumin.     (Miescher,  1874.) 

Nucleo-albumin,  a  nuclein  having  a  relatively  high  percentage  of  albumin. 
Distinguished  from  nucleo-proteids  by  containing  paranucleic  acid  which  yields 
no  xantl>in-bodies. 

Nucleochyrema  (x^A-o's,  juice),  the  ground-substance  of  the  nucleus  as  opposed 
to  that  of  the  cytoplasm.     (Strasburger,  1882.) 

Nucleohy'aloplasma  (see  Hyaloplasm),  the  achromatic  substance  (linin)  in 
which  the  chromatin-granules  are  suspended.     (Strasburger,  1882.) 

Nucleomi'crosomes  (see  Microsome),  the  nuclear  (chromatin)  granules  as 
opposed  to  those  of  the  cytoplasm.     (Strasburger,  1882.) 

Nu'cleoplasm.  i.  The  reticular  substance  of  the  (egg-)  nucleus.  (Van 
Beneden,  1875.)  ^-  Th^  substance  of  the  nucleus  as  opposed  to  that  of  the 
cell-body  or  cytoplasm.     (Strasburger,  1882.)  » 

Nuoleo-pro'teid,  a  nuclein  having  a  relatively  high  percentage  of  albumin.  May 
be  split  into  albumin  and  true  nucleic  acid,  the  latter  yielding  xanthin-bodies. 

CEdematin  (otSry/xa,  a  swelling),  the  granules  or  microsomes  of  the  nuclear  ground- 
substance.     (Reinke,  1893.) 

O'ocyte  (Ovocyte),  (ojoi/,  egg;  Kvro<;,  hollow  (a  cell)),  the  ultimate  ovarian  egg 
before  formation  of  the  polar  bodies.  The  primary  oocyte  divides  to  form  the 
first  polar  body  and  the  secondary  oocyte.  The  latter  divides  to  form  the  second 
polar  body  and  the  mature  egg.     (Boveri,  1891.) 

Oogen'esis,  Ovogenesis  (ww,  egg;  yeVecrt?,  origin),  the  genesis  of  the  egg  after 
its  origin  by  division  from  the  mother-cell.  Often  used  more  specifically  to 
denote  the  process  of  reduction  in  the  female. 

Oogo'nium,  Ovogonium  (wdv,  egg ;  yovrj,  generation),  i.  The  primordial  mother- 
cell  from  which  arises  the  egg  and  its  follicle.  (Pfluger.)  2.  The  descendants 
of  the  primordial  germ-cell  which  ultimately  give  rise  to  the  oocytes  or  ovarian 
eggs.     (Boveri,  1891.) 

Ookine'sis  (wov,  egg:,  Kcvrja-t^,  movement),  the  mitotic  phenomena  of  the  egg  dur- 
ing maturation  and  fertilization.     (Whitman,  1887.) 

O'voccntre,  the  egg-centrosome  during  fertilization.     (Fol,  1891.) 

Oxychro'matin  (o^u?,  acid  ;  see  Chromatin),  that  portion  of  the  nuclear  substance 
stained  by  acid  aniline  dyes.  Equivalent  to  "linin"  in  the  usual  sense. 
(Heidenhain,  1894.) 

Pangen'esis  (ttS?  (ttuv-)^  all;  yeVeo-t?,  production),  the  theory  of  gemmules,  ac- 
cording to  which  hereditary  traits  are  carried  by  invisible  germs  thrown  off  by 
the  individual  cells  of  the  body.     (Darwin,  1868.) 

Pangens  (ttSs  (Trav-),  all ;  -yevy<;,  producing),  the  hypothetical  ultimate  supra- 
molecular  units  of  the  idioplasm,  and  of  the  cell  generally.  Equivalent  to 
gemmules,  micellae,  idioblasts,  biophores,  etc.     (De  Vries,  1889.) 

Panmeri'stic  (Trav.  all;  /x€po<^,  part),  relating  to  an  ultimate  protoplasmic  structure 
consisting  of  independent  units.     See  Pangen. 

Parachro'matin  (see  Chromatin),  the  achromatic  nuclear  substance  (linin  of 
Schwarz)  from  which  the  spindle-fibres  arise.     (Pfitzner,  1883.) 

Parali'nin  (see  Linin),  the  nuclear  ground-substance  or  nuclear  sap.  (Schwarz, 
1887.) 

Parami'tome  (see  Mitome),  the  ground-substance  or  interfilar  substance  of  pro- 
toplasm, opposed  to  mitome.     (FlExMMING,  1892.) 

Paranu'clein  (see  Nuclein),  the  substance  of  true  nucleoli  or  plasmosomes. 
Pyrenin  of  Schwarz.  (O.  Hertwig,  1878.)  Applied  by  Kossel  to  "nucleins'' 
derived  from  the  cytoplasm.  These  are  compounds  of  albumin  and  paranucleic 
acid  which  yields  no  xanthin-bodies. 


340  GLOSSARY 

Par'aplasm  {Trapa,  beside;  TrXdafm,  something  formed),  the  less  active  portion  of 
the  cell-substance.  Originally  applied  by  Kupfter  to  the  cortical  region  of  the 
cell  (exoplasm),  but  now  often  applied  to  the  ground-substance.     (Kupffer, 

1875.) 

Periplast  (-n-epL,  around  ;  TrXao-Tos,  form),  a  term  somewhat  vaguely  applied  to  the 
attraction-sphere.  The  term  daughter-periplast  is  applied  to  the  centrosome. 
(Vejdovsky,  1888.) 

Plas'mosome  (7rAao-/xa,  something  formed  (/.e.  protoplasmic)  ;  alhfxa,  body),  the 
true  nucleolus,  distinguished  by  its  affinity  for  acid  anilines  and  other  "  plasma- 
stains."'     (Ogata,  1883.) 

Pla'some  (TrXdarfjua,  a  thing  formed;  o-oi/xa,  body),  the  ultimate  supra-molecular 
vital  unit.     See  Biophore,  Pangen.     (Wiesner,  1890.) 

Plas'tid  (TrXao-To?,  form),  i.  A  cell,  whether  nucleated  or  non-nucleated.  (HAckel, 
1866.)  2.  A  general  term  applied  to  permanent  cell-organs  (chloroplasts,  etc.) 
other  than  the  nucleus  and  centrosome.     (Schimper,  1883.) 

Plas'tidule,  the  ultimate  supra-molecular  vital  unit.  (Elssberg,  1874;  Hackel, 
1876.) 

Plas'tin,  a  term  of  vague  meaning  applied  to  a  substance  related  to  the  nucleo- 
proteids  and  nucleo-albumins  constituting  the  linin-network  (Zacharias)  and  the 
cytoreticulum  (Carnoy).     (Reinke  and  Rodewald,  1881.) 

Pluriva'lent  (ph^s,  more ;  va/ere,  to  be  worth),  applied  to  chromatin-rods  that 
have  the  value  of  more  than  one  chromosome  sensu  strictu.     (Hacker,  1892.) 

Polar  bodies  (Polar  globules),  two  minute  cells  segmented  oif  from  the  ovum 
before  union  of  the  germ-nuclei.  (Disc,  by  Carus,  1824;  so  named  by  Robin, 
1862.) 

Polar  corpuscle,  the  centrosome.     (Van  Beneden,  1876.) 

Polar  rays  (Polradien),  a  term  sometimes  applied  to  all  of  the  astral  rays  as 
opposed  to  the  spindle-fibres,  sometimes  to  the  group  of  astral  rays  opposite  to 
the  spindle-fibres. 

Pole-plates  (End-plates),  the  achromatic  spheres  or  masses  at  the  poles  of  the 
spindle  in  the  mitosis  of  Protozoa,  probably  representing  the  attraction-spheres. 
(R.  Hertwig.  1877.) 

Polyspermy,  the  entrance  into  the  ovum  of  more  than  one  spermatozoon. 

Prochro'matin  (see  Chromatin),  the  substance  of  true  nucleoli,  or  plasmosomes. 
Equivalent  to  paranuclein  of  O.  Hertwig.     (Pfitzxer,  1883.) 

Pronuclei,  the  germ-nuclei  during  fertilization;  /.^.,  the  egg-nucleus  (female  pro- 
nucleus) after  formation  of  the  polar  bodies,  and  the  sperm-nucleus  (male  pro- 
nucleus) after  entrance  of  the  spermatozoon  into  the  ^gg.  (Van  Beneden, 
1875.) 

[Prothy'alosome]  (see  Hyalosome),  an  area  in  the  germinal  vesicle  (of  Ascaris) 
by  which  the  germinal  spot  is  surrounded,  and  which  is  concerned  in  formation 
of  the  first  polar  body.     (Van  Beneden,  1883.) 

Pro'toblast  (Trpwros,  first ;  ^Aao-ros,  a  germ),  a  naked  cell,  devoid  of  a  membrane. 
(Kolliker.) 

Pro'toplasm  (7rpa»T05.  first;  7rAa(T/xa,  a  thing  formed  or  moulded),  i.  The  living 
substance  of  the  cell,  comprising  cytoplasm  and  karyoplasm.  (Purkyne,  1840; 
H.  VON  Mohl,  1846.)     2.  The  cytoplasm  as  opposed  to  the  karyoplasm. 

Pro'toplast  (TrpcoTos,  first;  TrAao-ros,  formed),  i.  The  protoplasmic  body  of  the 
cell,  including  nucleus  and  cytoplasm,  regarded  as  a  unit.  Nearly  equivalent  to 
the  energid  of  Sachs.  (Hanstein,  1880.)  2.  Used  by  some  authors  synony- 
mously with  plastid. 

[Pseudochro'matin]  (see  Chromatin),  the  same  as  prochromatin.  (Pfitzner, 
1886.) 


GLOSSARY  341 

Pseudonu'clein  (see  Nuclein),  the  same  as  the  paranuclein  of  Kossel.     (Ham- 

MARSTEN,   1894.) 

Pseudo-reduction,  the  preHminary  halving  of  the  number  of  chromatin-rods  as  a 
prekide  to  the  formation  of  the  tetrads  and  to  the  actual  reduction  in  the  number 
of  chroiiiosomes  in  maturation.     (ROckert,  1894.) 

Pyre'nin  {irvprfv,  the  stone  of  a  fruit;  i.e.  relating  to  the  nucleus),  the  substance 
of  true  nucleoli.     Equivalent  to  the  paranuclein  of  Hertwig.     (Schwarz,  1887.) 

Pyre'noid  {Trvprjv,  the  stone  of  a  fruit;  like  a  nucleus),  colourless  plastids  (leuco- 
plastids),  occurring  in  the  chromatophores  of  lower  plants,  forming  centres  for 
the  formation  of  starch.     (Schmitz,  1883.) 

Reduction,  the  halving  of  the  number  of  chromosomes  in  the  germ-nuclei  during 
maturation. 

Sertoli-cells,  the  large,  digitate,  supporting,  and  nutritive  cells  of  the  mammalian 
testis  to  which  the  developing  spermatozoa  are  attached.  (Equivalent  to  "  sper- 
matoblast^' as  originally  used  by  von  Ebner,  1871.) 

Sper'matid  (o-Tre'p/xa,  seed),  the  final  cells  which  are  converted  without  further 
division  into  spermatozoa  ;  they  arise  by  division  of  the  secondary  spermatocytes 
or  "  Samenmiitterzellen."     (La  Valette  St.  George,  1886.) 

Sper'matoblasts  (o-Tre'/o/xa,  seed;  ^Aao-ros,  germ),  a  word  of  vague  meaning, 
originally  applied  to  the  supporting  cell  or  Sertoli-cell,  from  which  a  group  of 
spermatozoa  was  supposed  to  arise.  By  various  later  writers  used  synonymously 
with  spermatid,     (von  Ebner,  1871.) 

Sper'matocyst  (o-Trepyua,  seed ;  kvcttis,  bladder),  originally  applied  to  a  group  of 
sperm-producing  cells  ("spermatocytes ''),  arising  by  division  from  an  "  Ursa- 
menzelle"  or  "spermatogonium."     (La  Valette  St.  George,  1876.) 

Sper'matocyte  ((nrep/xa,  seed;  kvtos,  hollow  (a  cell)),  the  cells  arising  from  the 
spermatogonia.  The  primary  spermatocyte  arises  by  growth  of  one  of  the  last 
generation  of  spermatogonia.  By  its  division  are  formed  two  secondary  sper^ 
matocytes^  each  of  which  gives  rise  to  two  spermatids  (ultimately  spermatozoa). 
(La  Valette  St.  George,  1876.) 

[Spermatogem'ma]  (o-7rep/xa,  seed;  gemma,  bud),  nearly  equivalent  to  spermato- 
cyst.  Differs  in  the  absence  of  a  surrounding  membrane.  [In  mammals, 
La  Valette  St.  George,  1878.] 

Spermatogen'esis  (airep/xa,  seed ;  yeVetn?,  origin),  the  phenomena  involved  in 
the  formation  of  the  spermatozoon.  Often  used  more  specifically  to  denote  the 
process  of  reduction  in  the  male. 

Spermatogo'nium  ("  Ursamenzelle " )  ((nrepfxa,  seed;  yovr/,  generation),  the 
descendants  of  the  primordial  germ-cells  in  the  male.  Each  ultimate  sper- 
matogonium typically  gives  rise  to  four  spermatozoa.  (La  Valette  St. 
George.  1876.) 

Sperniatome'rites  (cnripjxa,  seed;  p.ipo^y  a  part),  the  chromatin-granules  into 
which  the  sperm-nucleus  resolves  itself  after  entrance  of  the  spermatozoon.  (In 
Petroinyzon,  Bohm,  1887.) 

Sper'matosome  ((nrepfjia,  seed ;  crw/xa,  body),  the  same  as  spermatozoon.  (La 
Valette  St.  George,  1878.) 

Spermatozo'id  (see  Spermatozoon),  the  ciliated  paternal  germ-cell  in  plants. 
The  word  was  first  used  by  von  Siebold  as  synonymous  with  spermatozoon. 

Spermatozo'on  (crTrep/xa,  seed;  ^wov,  animal),  the  paternal  germ-cell  of  animals. 
(Leeuweinhoek,  1677.) 

Sperm-nucleus,  the  nucleus  of  the  spermatozoon  ;  more  especially  applied  to  it  after 
entrance  into  the  egg  before  its  union  with  the  egg-nucleus.  In  this  sense 
equivalent  to  the  "male  pronucleus"  of  Van  Beneden.     (O.  Hertwig,  1875.) 

Sper'mocentre,  the  sperm-centrosome  during  fertilization.     (Fol,  1891.) 


342  GLOSSARY 

Spi'reme  (<nreipr}fJLa,  a  thing  wound  or  coiled ;  a  skein),  the  skein  or  "Knauel" 

stage  of  the  nucleus  in  mitosis,  during  which  the  chromatin  appears  in  the  form 

of  a  thread,  continuous  or  segmented.     (Flemming,  1882.) 
Spon'gioplasm  {aTroyyiov,  a  sponge;  Tr\d<Tfjia,  a  thing  formed),  the  cytoreticulum. 

(Leydig,  1885.) 
Ste'reoplasm  (o-repeds,  solid),  the  more  solid  part  of  protoplasm  as  opposed  to  the 

more  fluid  "  hygroplasm."     (Nageli,  1884.) 
Substantia    hyalina,    the     protoplasmic     ground-substance     or     "hyaloplasm*."" 

(Leydig,  1885.) 
Substantia  opaca,  the  protoplasmic   reticulum  or   "  spongioplasm."       (Leydig, 

1885.) 
Te'loblast  (tcXos,  end;  (Skaa-ros,  a  germ),  large  cells  situated  at  the  growing  end 

of   the   embryo    (in   annelids,  etc.),  which    bud  forth    rows   of  smaller  cells. 

(Whitman,  Wilson,  1887.) 
Telole'cithal  (re'Aos,  end;  AcKt^os,  yolk),  that  type  of  ovum  in  which  the  yolk  is 

mainly  accumulated  in  one  hemisphere.     (Balfour,  1880.) 
Telophases,   Telokine'sis   (tcAos,  end),   the  closing   phases   of   mitosis,  during 

which  the  daughter-nuclei  are  re-formed.     (Heidenhain,  1894.) 
To'noplasts  (rovoq,  tension  ;  TrXaaros,  form),  plastids  from  which  arise  the  vacuoles 

in  plant-cells.     (De  Vries,  1885.) 
Trophoplasm    {rpoK^rj,   nourishment ;  TrXda-fxa) .       i .   The   nutritive    or   vegetative 

substance  of  the  cell,  as   distinguished  from   the   idioplasm.     (Nageli,   1884.) 

2.  The   active   substance   of  the   cytoplasm    other   than  the    "  kinoplasm "   or 

archoplasm.     (Strasburger,  1892.) 
Tro'phoplasts  {Tpo<f>rj.  nourishment;  TrAao-rds,  form),  a  general  term,  nearly  equiv- 
alent  to    the    "plastids"    of    Schimper,    including  "  anaplasts "   (amyloplasts). 

*' autoplasts  "  (chloroplasts),  and  chromoplasts.     (A.  Meyer,  1882-83.) 
Yolk-nucleus,  a  word  of  vague  meaning  applied  to  a  cytoplasmic  body,  single  or 

multiple,  that  appears  in  the  ovarian  egg.     [Named  "  Dotterkern  "  by  Carus, 

1850.] 
Zy'gote  or  Zy'gospore  {^vyov,  a  yoke),  the  cell  produced  by  the  fusion  of  two 

conjugating  cells  or  gametes  in  some  of  the  lower  plants. 


GENERAL    LITERATURE-LIST 

The  following  list  includes  only  the  titles  of  works  actually  referred  to  in  the  text 
and  those  immediately  related  to  them.  For  more  complete  bibliography  the  reader 
is  referred  to  the  literature-lists  in  the  special  works  cited,  especially  the  following. 
For  reviews  of  the  early  history  of  the  cell-theory  see  Remak's  Untersuchungen 
(!50-'55),  Huxley  on  the  Cell-theory  ('53),  and  Tyson's  Cell-doctrine  C78).  An 
exliaustive  review  of  the  earlier  literature  on  protoplasm,  nucleus,  and  cell- 
division  will  be  found  in  Flemming's  Zellsubstanz  ('82),  and  a  later  review  of 
theories  of  protoplasmic  structure  in  Blitschli's  Protoplasjna  ('92).  The  earlier 
work  on  mitosis  and  fertilization  is  very  thoroughly  reviewed  in  Whitman's  Clep- 
sine  ('78),  FoPs  Hhiogenie  ('79),  and  Mark's  Limax  ('81).  For  more  recent 
general  literature-lists  see  especially  Hertwig's  Zelle  mid  Gewebe  ('93),  Yves 
Delage  ('95),  Henneguy's  Cellule  ('96),  and  the  admirable  reviews  by  Flemming, 
Boveri,  Ruckert,  Roux,  and  others  in  Merkel  and  Bonnet's  Ergebnisse  ('9i-'94). 

The  titles  are  arranged  in  alphabetical  order,  according  to  the  system  adopted  in 
Minot's  Humaji  E)nbryology.  Each  author's  name  is  followed  by  the  date  of  publi- 
cation (the  first  two  digits  being  omitted,  except  in  case  of  works  published  before 
the  present  century),  and  this  by  a  single  number  to  designate  the  paper,  in  case 
two  or  more  works  were  published  in  the  same  year.  For  example,  Boveri,  Th., 
'87,  2,  denotes  the  second  paper  published  by  Boveri  in  1887. 

In  order  to  economize  space,  the  following  abbreviations  are  used  for  the  journals 
most  frequently  referred  to  :  — 

ABBREVIATIONS 

A.  A.     Anatomischer  Anzeiger.  > 

A.  B.     Archives  de  Biologic. 

A.  A.  P.  Archiv  fur  Anatomic  und  Physiologic. 

A.  in.  A.  Archiv  fiir  mikroscopische  Anatomic. 

A.  Entm,  Archiv  fiir  Entwicklungsmechanik. 

B.  C.  Biologischcs  Centralblatt. 

C.  R.     Comptcs  Rendus. 

J.  M.  Journal  of  Morphology. 

J.  Z:  Jenaische  Zeitschrift. 

M.  A.  Miillcr's  Archiv, 

M.J.  Morphologisches  Jahrbuch. 

Q.  J.  Quarterly  Journal  of  Microscopical  Science. 

Z.  A.  Zoologischer  Anzeiger. 

Z.  %v.  Z.  Zeitschrift  fiir  wissenschaftliche  Zoologie. 

ACQUA,  '91.  Contribuzione  alia  conoscenza  della  cellula  vegetale  :  Malpighia, 
V.  —  Altman,  R.,  '86.  Studien  iiber  die  Zelle.  I. :  Leipzig.  —  Id.,  '87.  Die  Genese 
der  Zellen:  Leipzig. — Id.,  '89.  Uber  Nucleinsaure :  A.  A.  P.,  p.  524. —  Id., 
'90,  '94.  Die  Elementarorganismen  und  ihre  Beziehung  zu  den  Zellen  :  Leipzig.  — 
Amelung,  E.,  '93.     Uber  mittlere  Zellgrosse  :  Elora,  p.    176. — Arnold,  J.,  '79. 

343 


3  44  GENERAL  LIT  ERA  T  URE-LIS  T 

tlber  feinere  Struktur  der  Zellen,  etc.  :  Virchow's  Arch.,  1879.     (See  earlier  papers.) 

—  Auerbach,  L.,  '74.  Organologische  Studien  :  Breslau.  —  Id.,  '91.  liber  einen 
sexuellen  Gegensatz  in  der  Chromatophilie  der  Keimsubstanzen :  Sitziingsber.  der 
Konigl.  preuss.  Akad.  d.  Wiss.  Berlin,  XXXV. 

VON  BAER,  C.  E.,  '28,  '37,  tlber  Entwickelungsgeschichte  der  Thiere.  Beo- 
bachtung  und  Reflexion:  I.  Konigsberg,  1828;  II.  1837.  — Id.,  '34.  Die  Metamor- 
phose des  Eies  der  Batrachier :  Muller's  Arch. — Balbiani,  E.  G.,  '64.  Sur  la 
constitution  du  germe  dans  I'oeuf  animal  avant  la  fecondation :  C.  R.,  LVIII. — 
Id.,  '76.  Sur  les  phenom^nes  de  la  division  du  noyau  cellulaire  :  C.  R  ,  XXX.,  Octo- 
ber, 1876.  —  Id.,  '81.  Sur  la  structure  du  noyau  des  cellules  salivares  chez  les  larves 
de  Chironomus  :  Z.  A.,  1881,  Nos.  99,  100.  —  Id.,  '89.  Recherches  expdrimen- 
tales  sur  la  merotomie  des  Infusoires  cilids :  Recueil  Zool.  Suisse,  January,  1889.  — 
Id.,  '91,  1.  Sur  les  rdgendrations  successives  du  peristome  chez  les  Stentors  et  sur 
le  role  du  noyau  dans  ce  phenomene  :  Z.  A.,  372,  373.  —  Id.,  '91,  2.  Sur  la  struc- 
ture et  division  du  noyau  chez  les  Spirochona  gemmipara :  Ann.  d.  Micrographie. 
Id.,  '93.     Centrosome  et  Dotterkern  :  Joiirn.  de  Vaiiat.  et  de  la  physiol.,  XXIX.  — 

—  Balfour,  F.  M.,  '80.  Comparative  Embryology:  I.  1880.  —  Ballowitz,  '88-'91. 
Untersuchungen  liber  die  Struktur  der  Spermatozoen :  i,  (birds)  A.  m.  A.,  XXXII., 
1888;  2.  (insects)  Z.  iv.  Z.,  LX.,  1890;  3.  (fishes,  amphibia,  reptiles)  A.  ?n.  A. 
XXXVI.,  1890;  4.  (mammals)  Z.  w.  Z.,  1891. — Id.,  '89.  Fibrillare  Struktur  und 
Contractilitat :  Arch.  ges.  Phys.,  XLVI.  —  Id., '91,  2.  Die  innere  Zusammensetz- 
ung  des  Spermatozoenkopfes  der  Saugetiere  :  Centralb.  f.  Phys.,  V.  —  Id.,  '95. 
Die  Doppelspermatozoa  der  Dytisciden  :  Z.  w.  Z.,  XLV.,  3.  —  Van  Bambeke,  C, 
'93.  Elimination  d'eldments  nucleaires  dans  I'oeuf  ovarien  de  Scorpaena  scrofa : 
A.  B.,  XIII.  I.  — De  Bary,  '58.  Die  Conjugaten.  —  Id.,  '62.  (Jber  den  Bau 
und  dasWesen  der  Zelle  :  Flora,  1862. — Id.,  '64.  Die  Mycetozoa  :  2d  Ed.,  Leip- 
zig. —  Barry,  M.  Spermatozoa  observed  within  the  Mammiferous  Ovum :  Phil. 
Trans.,  1843.  —  Beale,  Lionel  S.,  '61.  On  the  Structure  of  Simple  Tissues  of  the 
Human  Body:  London.  —  B^champ  and  Estor, '82.  De  la  constitution  elemen- 
taire  des  tissues  :  Montpellier.  —  Belajeff, '94,  1.  Zur  Kenntniss  der  Karyokinese 
bei  den  Pflanzen :  Flora,  1894,  Erganzungsheft. — Id.,  '94,  2.  Uber  Bau  und 
Entwicklung  der  Spermatozoiden  der  Pflanzen:  Flora,  LIV.  —  Benda,  C,  '87. 
Untersuchungen  iiber  den  Bau  des  funktionirenden  Samenkenkalchens  einiger  Sau- 
gethiere :  A.  tfi.  A.  —  Id.,  '93.  Zellstrukturen  und  Zelltheilungen  des  Salaman- 
derhodens :  Verh.  d.  Anal.  Ges.,  1893. — Van  Beneden,  E.,  '70.  Recherches 
sur  la  composition  et  la  signification  de  I'oeuf:  Mem.  cour.  de  VAc.  roy.  d.  S.  de 
Belgique,  1870. — Id.,  '75.  La  maturation  de  I'oeuf,  la  fecondation  et  les  premieres 
phases  du  d^veloppement  embryonnaire  des  mammiferes  d'apres  des  recherches 
faites  chez  le  lapin  :  Bull.  Ac.  roy.  de  Belgique,  XI.  —  Id.,  '76,  1.  Recherches 
sur  les  Dicyemides:  Bull.  Acad.  Roy.  Belgique,  XLI.,  XLII.  — Id.,  '76,  2. 
Contribution  a  I'histoire  de  la  vesicule  germi native  et  du  premier  noyau  embryon- 
naire:  Ibid.,  XLI.;  also  g./.,  XVI.  —  Id., '83.  Recherches  sur  la  maturation  de 
I'ceuf,  la  fecondation  et  la  division  cellulaire:  A.  B.,  IV.  —  Van  Beneden  and 
Julin,  *84,  1.  La  segmentation  chez  les  Ascidiens  et  ses  rapports  avec  I'organi- 
sation  de  la  larve :  Ibid.,  V.  —  Id.,  '84,  2.  La  spermatogenese  chez  I'Ascaride 
m^galoc^phale :  Bull.  Acad.  Roy.  Belgique,  3me  ser.,  VII.  —  Van  Beneden,  E., 
et  Neyt,  A.,  '87.  Nouvelles  recherches  sur  la  fecondation  et  la  division  mitosique 
chez  I'Ascaride  mdgalocephale  :  Ibid.,  1887.  —  Bergh,  R.  S.,  '94.  Vorlesungen  i.iber 
die  Zelle  und  die  einfachen  Gewebe  :  Wiesbaden. — Id., '95.  (Iber  die  relativen 
Theilungspotenzen  einiger  Embryonalzellen  :  A.  Fntm.,  II.,  2.  — Bernard,  Claude. 
Le<;ons  sur  les  Phdnomtines  de  la  Vie:  ist  Ed.  1878,  2d  Ed.  1885,  Paris.  —  Ber- 
thold,  G.,  '86.     Studien  iiber  Protoplasma-mechanik  :  Leipzig.  —  Bickford,  E.  E., 


GENERAL   LITERATURE-LIST  345 

'94.  Notes  on  Regeneration  and  Heteromorphosis  of  Tubularian  Hydroids :  /.  M.^ 
IX.,  3.  — Biondi,  D.,  '85.     Die  Entwicklung  der  Spermatozoiden  :  A.  in.  A.,  XXV. 

—  Blanc,  H.,  '93.  Etude  sur  la  fecondation  de  Toeuf  de  la  truite :  Ber.  Natiir- 
forsch.  Ges.  sit  Freiburg,  VIII.  —  Blochmann,  F.,  '87,  2.  tJber  die  Richtungs- 
korper  bei  ^nsekteneiern  :  M. /.,  XII.  — Id., '88.  tJber  die  Richtungskorper  bei 
unbefruchtet  sich  entvvickelnden  Insekteneiern  :  Verh.  naturh.  ined.  Ver.  Heidel- 
berg, N.  F.,  IV.,  2.  —  Id.,  '89.  Uber  die  Zahl  der  Richtungskorper  bei  befruchteten 
und  unbefruchteten  Bieneneiern  :  M.  J.  —  Id.,  '94.  Uber  die  Kerntheilung  bei 
Euglena  :  B.  C,  XIV.  —  Bolim,  A.,  '88.  tJber  Reifung  und  Befruchtung  des  Eies 
von  Petromyzon  Planeri :  A.  m.  A.,  XXXII.  —  Id.,  '91.  Die  Befruchtung  des 
Forelleneies :  Sitz.-Ber.  d.  Ges.f.  Morph.  u.  Phys.  Munchen,  VII.  —  Boll,  Fr.,  '76. 
Das  Princip  des  Wachsthums  :  Berlin.  —  Bonnet,  C,  1762.  Considerations  sur 
les  Corps  organises:  Anisterdain.  —  Born,  G.,  '85.  Uber  den  Einfluss  der 
Schwere  auf  das  Froschei :  A.  m.  A.,  XXIV. — Id..  '94,  Die  Structur  des  Keim- 
blaschens  im  Ovarialei  von  Triton  taeniatus  :  A.  in.  A.,  XLIII.  —  Bourne,  G.  C, 
'95.  A  Criticism  of  the  Cell-theory  ;  being  an  answer  to  Mr.  Sedgwick's  Article  on 
the  Inadequacy  of  the  Cellular  Theory  of  Development:  Q.  J.,  XXXVIII.,  i.— 
Boveri,  Th.,  *86.  tJber  die  Bedeutung  der  Richtungskorper:  Sitz.-Ber.  Ges. 
Morph.  u.  Phys.  Miinchen,  II.  —  Id.,  '87,  1.  Zellenstudien,  Heft  I. :  J.  Z.,  XXI.  — 
Id.,  '87,  2.  ttber  die  Befruchtung  der  Eier  von  Ascaris  rnegalocephala :  Sitz.-Ber. 
Ges.  Morph.  Phys.  Miinchen,  III.  — Id.,  '87,  2.  Uber  den  Anteil  des  Spermatozoon 
an  der  Teilung  des  Eies:  Sitz.-Ber.  Ges.  Morph.  Phys.  Miinchen,  III.,  3.  —  Id., 
'87,  3.  Uber  Differenzierung  der  Zellkerne  wahrend  der  Furchung  des  Eies  von 
Ascaris  meg.:  A.  A.,  1887.— Id.,  '88,  1.  Uber  partielle  Befruchtung:  Sitz.-Ber. 
Ges.  Morph.  Phys.  Miinchen,  IV.,  2.— Id.,  '88,  2.  Zellenstudien,  II.:  J.  Z., 
XXII.  —  Id.,  '89.  Ein  geschlechtlich  erzeugter  Organismus  ohne  miitterliche 
Eigenschaften :  Sitz.-Ber.  Ges.  Morph.  Phys.  Miinchen,  V.  Trans,  in  Am.  Nat., 
March,  '93.— Id.,  '90.  Zellenstudien,  Heft  III.:  J.  Z.,  XXIV. —Id., '91.  Be- 
fruchtung: Market  und  Boniiefs  Ergebnisse,  I. — Id.,  '95,  1.  Uber  die  Befruch- 
tungs-und  Entwickelungsfahigkeit  kernloser  Seeigel-Eier,  etc:  A.  Ejitni.,  II.,  3. — 
Id.,  '95,  2.  liber  das  Verhalten  der  Centrosomen  bei  der  Befruchtung  des  Seeige- 
leies,  nebst  allgemeinen  Bemerkungen  Uber  Centrosomen  und  Verwandtes  :  Verh.  d. 
Physikal.-nied.  Gesellschaft  zu  Wiirzburg,  N.  F.,  XXIX.,  i. — Braem,  F.,  '93. 
Des  Prinzip  der  organbildenden  Keimbezirke  und  die  entwicklungsmechanischen 
Studien  von  H.  Driesch  :  B.  C.,  XIII.,  4,  5.  — Brandt,  H.,  '77.  Uber  Actino- 
sphaerium  Eichhornii :  Dissertation,  Halle,  iZ']'] . — Brass,  A., '83-4.  Die  Organi- 
sation der  thierischen  Zelle :  Halle.  —  Brauer,  A., '92.  Das  Ei  von  Branchipus 
Grubii  von  der  Bildung  bis  zur  Ablage  :  Abh.preuss.  Akad.  Wiss.,  '92.  —  Id.,  '93, 1. 
Zur  Kenntniss  der  Reifung  des  parthenogenetisch  sich  entwickelnden  Eies  von 
Artemia  Salina :  A.  m.  A.,  XLIII. — Id.,  '93,  2.  Zur  Kenntniss  der  Spermato- 
genese  von  Ascaris  megalocephala :  A.  ni.  A.,  XLII.  —  Id., '94.  Uber  die  En- 
cystierung  von  Actinosphoerium  Eichhornii:  Z.  w.  Z.,  LVIIL,  2.  —  Braus,  '95. 
Uber  Zellteilung  und  Wachstum  des  Tritoneies:  J.  Z.,  XXIX.  — Brooks,  W.  K., 
'83.  The  Law  of  Heredity:  Baltimore.— 'Brov^n,  H.  H.,  '85.  On  Spermato- 
genesis in  the  Rat:  Q.  J.,  XXV. —  Brown,  Robert,  '33.  Observations  on  the 
Organs  and  Mode  of  Fecundation  in  Orchideae  and  Asclepiadeae :  Trans.  Linn. 
Soc,  1833. — Brunn,  M.  von,  '89.  Beitrage  zur  Kenntniss  der  Samenkbrper 
und  ihrer  Entwickelung  bei  Vogeln  und  Saugethieren :  A.  m.  A.,  XXXIII. — 
Brucke,    C.    '61.      Die    Elementarorganismen :     Wiener  Sitzber.,   XLIV.,    1861. 

—  Biirger,  O.,  '91.  tJber  Attractionsspharen  in  den  Zellkorpern  einer  Leibes- 
fliissigkeit:  A.  A.,  VI.  —  Id.,  92.  Was  sind  die  Attractionsspharen  und  ihre 
Centralkorper  ?  A.  A..  1892.  —  De  Bruyne,  C,  '95.  La  sphere  attractive  dans 
les    cellules    fixes    du    tissu    conjonctif:    Bidl.    Acad.    Sc.   de    Belgique,    XXX. — 


34b  GENERAL   LITERATURE-LIST 

Butschli,  O.,  '73.  Beitrage  zur  Kenntniss  der  freilebenden  Nematoden  :  Nova  acta 
acad.  Car.  Leopold,  XXXVI.  — Id.,  '75.  Vorlaufige  Mitteilungen  iiber  Unter- 
suchungen  betreflfend  die  ersten  Entwickelungsvorgange  ini  befmchteten  Ei  von 
Nematoden  und  Schnecken :  Z.  w.  Z.,  XXV.  —  Id.,  '76.  Studien  liber  die  ersten 
Entwickelungsvorgange  der  Eizelle,  die  Zeliteilung  und  die  Konjugation  der  Infu- 
sorien :  Abh.  des  Senckenb.  Naturforscher-Ges.^  X.  —  Id.,  '91.  Uber  die  soge- 
nannten  Centralkorper  der  Zellen  und  ihre  Bedeutung:  Verh.  Naticrhist.  Med.  Ver. 
Heidelbe7'g,  1891. — Id.,  '92,  1.  tjber  die  klinstliche  Nachahmung  der  Karyoki- 
netischen  Figuren :  Ibid.,  N.  F.,  V. — Id.,  '92,  2.  Untersuchungen  iiber  mikro- 
skopische  Schaume  und  das  Protoplasma  (full  review  of  literature  on  protoplasmic 
structure):  Leipzig  {E?igelmajin).  —  Id.,  '94.  Vorlaufige  Bericht  iiber  fortgesetzte 
Untersuchungen  an  Gerinnungsschaumen,  etc. :   Verh.  Naturhist.  Ver.  Heidelberg,  V. 

CALKINS,  G.  N.,  '95,  1.  Observations  on  the  Yolk-nucleus  in  the  Eggs  of 
Lumbricus  :  Trans.  N.Y.  Acad.  Sci.,  June,  1895. — Id., '95,  2.  The  Spermato- 
genesis of  Lumbricus  :  J.  M.,  XL,  2.  —  Carnoy,  J.  B.,  '94.  La  biologic  cellulaire  : 
Li^ge.  —  Id., '85.  La  cytodierese  des  Arthropodes  :  La  Cellule,  \.  —  Id., '86.  La 
cytodidrese  de  I'oeuf:  La  Cellide,\\\.  —  Id., '86.  La  vdsicule  germinative  et  les 
globules  polaires  chez  quelques  Nematodes:  La  Cellule,  IIL  —  Id., '86.  La  seg- 
mentation de  I'oeuf  chez  les  Nematodes  :  La  Cellule,  IIL,  i.  —  Calberla.  E.,  '78.  Der 
Befruchtungsvorgang  beim  Ei  von  Petromyzon  Planeri :  Z.  w.  Z.,  XXX.  —  Campbell, 

D.  H.,  '88-9.  On  the  Development  of  Pilularia  globulifera :  Auft.  Bot.,  II. — 
Castle,  W.  E.,  '96.  The  Early  Embryology  of  Ciona  intestinalis  :  Bull.  Mus.  Coinp. 
Zocil.,  XKVll.,  7. —  Chabry,  L., '87.  Contributions  a  I'embryologie  normale  et 
pathologique  des  ascidies  simples:  Paris,  1887.  —  Chittenden,  R.  H.,  '94.  Some 
Recent  Chemico-physiological  Discussions  regarding  the  Cell :  Auu.  Nat.,  XXVII L, 
Feb.,  1894.  —  Chun,  C,  '90.  Uber  die  Bedeutung  der  direkten  Zelltheilung :  Sitzb. 
Schr.  Fhysik.-Okon.  Ges.  Konigsberg,  1890. — Id.,  '92,  1.  Die  Dissogonie  der 
Rippenquallen  :  Festschr.f.  Leuckart,  LMpzig,  1892. — Id.,  '92,  2.  (In  Roux,  '92, 
p.  55):  Verh.  d.  Afiat.  Ges.,  VL,  1892. — Clapp,  C.  M., '91.  Some  Points  in 
the  Development  of  the  Toad-Fish:  J.  M.,  V.  — Clarke,  J.  Jackson,  '95.  Ob- 
servations on  various  Sporozoa:  Q.  J.,  XXXVIL,  3. — Cohn,  Ferd.,  '51.  Nach- 
trage  zur  Naturgeschichte  des  Protococcus  pluvialis :  Nova  Acta,  XXII.  —  Conklin, 

E.  G.,  "94.  The  Fertilization  of  the  Ovum :  Biol.  Led.,  Marine  Biol.  Lab.,  Wood''s 
Holl,  Boston,  1894.  —  Id., '96.  Cell-size  and  Body-size:  Rept.  of  Am.  Morph. 
Sac,  Science  AW. ^]'3cci.  10,  1896.  —  Crampton,  H.  E., '94.  Reversal  of  Cleavage 
in  a  Sinistral  Gasteropod  :  Ann.  N.  Y.  Acad.  Sci.,  March,  1894. — Crampton  and 
"Wilson,  '96.  Experimental  Studies  on  Gasteropod  Development  (H.  E.  Cramp- 
ton).    Appendix  on  Cleavage  and  i\Iosaic-Work  (E.  B.  Wilson)  :  A.  Entm.,  IIL,  i. 

DEL  AGE,  YVES,  '95.  La  Structure  du  Protoplasma  et  les  Theories  sur  I'he're- 
ditd  et  les  grands  Problemes  de  la  Biologic  Generale :  Paris,  1895.  —  Demoor,  J., 
'95.  Contribution  a  I'etude  de  la  physiologic  de  la  cellule  (independance  fonction- 
elle  du  protoplasme  et  du  noyau)  :  A.  B.,  XIII.  —  Dogiel,  A.  S.,  '90.  Zur  Frage 
iiber  das  Epithel  der  Harnblase  :  A.  7n.  A.,  XXXV.  —  Driesch,  H.  Entwicklungs- 
mechanische  Studien;  I.,  II.,  1892,  Z.w.Z.,  LIIL;  III.-VL,  1893,  Ibid.,lN.',  VII.- 
X.,  1893:  Mitt.  Zool.  St.  Neapel,  XL,  2.— Id.,  '94.  Analytische  Theorie  der 
organischen  Entwicklung  :  Leipzig.  — Id.,  '95,  1.  Von  der  Entwickelung  einzelner 
Ascidienblastomeren  :  A.  Entm. A.,  3.  —  Driesch  and  Morgan,  '95,  2.  Zur  Analysis 
der  ersten  Entwickelungs  stadien  des  Ctenophoreneies :  Ibid.,  II.,  2. — Druuer, 
L.,  '94.  Zur  Morphologic  der  Centralspindel :  7.  Z.,  XXVIII.  (XXI.).— Id.,  '95. 
Studien  iiber  den  Mechanismus  der  Zelltheilung:  Ibid.,  XXIX..  2.  —  Diising,  C, 
'84.     Die  Regulierung  des  Geschlechtsverhaltnisses  :  Jena,  1884. 


GENERAL   LIT ERATU RE-LIST  34/ 

VON  EBNER,  V.,  '71.  Untersuchungen  iiber  den  Bau  der  Samencanalchen  und 
die  Entwicklung  der  Spermatozoiden  bei  den  Saugethieren  und  beim  Menschen  : 
l7ist.  Phys.  u.  Hist.  Graz.,  1871  {Leipzig). — Id.,  88.  Zur  Spermatogenese  bei 
den  Sauge^liieren :  A.  m.  A.,  XXXI.  —  Ehrlich,  P.,  '79.  Uber  die  specifischen 
Granulationen  des  Blutes :  A.  A.  I\  {Phys.),  1879,  P-  573- — Eismond,  J., '95. 
Einige  Beitrage  zur  Kenntniss  der  Attraktionsspharen  und  der  Centrosomen  :  A.  A., 
X. — Endres  and  'Walter,  '95.  Anstichversuche  an  Eiern  von  Rana  fusca:  A. 
Entm.,  II.,  I. — Engelmann,  T.  W., '80.  Zur  Anatomie  und  Physiologie  der 
Flimmerzellen  :  Arch.  ges.  Phys.,  XXIII.  —  von  Erlanger,  R.,  '96, 1.  Die  neuesten 
Ansichten  iiber  die  Zelltheilung  und  ihre  Mechanik :  Zo'ol.  Centralb.,  III.,  2.  —  Id., 
'96,  2.  Zur  Befruchtung  des  Ascariseies  nebst  Bemerkungen  liber  die  Struktur  des 
Protoplasmas  und  des  Centrosomes  :  Z.  A.,  XIX.  —  Id.,  '96,  3.  Neuere  Ansichten 
iiber  die  Struktur  des  Protoplasmas  :  Zo'ol.  Centralb.,  III.,  8,  9.  —  Errara,  '86.  Eine 
fundamentale  Gleichgevviclitsbedingung  organischen  Zellen :  Ber.  Deutsch.  Bot. 
Ges.,  1886. — Id.,  '87,  Zellformen  und  Seifenblasm  :  Tagebl.  der  60  Versammlimg 
dentscher  Natiirforscher  iind  Aerzte  zu  Wiesbaden,  1887. 

FARMER,  J.  B.,  '93.  On  nuclear  division  of  the  pollen-mother-cell  of  Lilium 
Martagon:  Ann.  Bot.,  WW.,  2T. — Id., '94.     Studies  in  Hepaticae:  Ibid.,  WW.,  2(). 

—  Id.,  '95,1.  tJber  Kernteilung  in  Lilium- Antheren,  besonders  in  Bezug  auf  die 
Centrosomenfrage  :  Flora,  1895,  p.  57.  —  Farmer  and  Moore,  '95,  On  the  essential 
similarities  existing  between  the  heterotype  nuclear  divisions  in  animals  and  plants : 
A.  A.,  XL,  3.  — Fick,  R.,  '93.  tJber  die  Reifung  und  Befruchtung  des  Axolotleies  : 
Z.  IV.  Z.,  LVL,  4. — Fiedler,  C,  '91.  Entwickelungsmechanische  Studien  an 
Echinodermeneier :  Festschr.  Ndgeli  ti.  Kolliker,  Zurich,  1891.  —  Field,  G.  "W.,  '95. 
On  the  Morphology  and  Physiology  of  the  Echinoderm  Spermatozoon  :  J.  M.,  XL  — 
Fischer,  A.,  '94.     Zur  Kritik  der  Fixierungsmethoden  der  Granula  :  A.  A.,  IX.,  22. 

—  Id.  '95,  Neue  Beitrage  zur  Kritik  der  Fixierungsmethoden:  Ibid.,X.. — Fleni- 
ming,  "W.,  '79.  Beitrage  zur  Kenntniss  der  Zelle  und  ihre  Lebenserscheinungen, 
I.:  A.  in.  A.,  XVL  —  Id.,  '79.  tJber  das  Verhalten  des  Kerns  bei  der  Zelltheilung, 
etc. :  Virchow's  Arch.,  LXXVII.  —  Id.,  '80.  Beitrage  zur  Kenntniss  der  Zelle  und 
ihrer  Lebenserscheinungen,  II. :  A.  m.  A.,  XIX.  — Id.,  '81.  Beitrage  zur  Kenntnis 
der  Zelle  und  ihrer  Lebenserscheinungen,  III. :  Ibid.,  XX.  — Id. ,'82.  Zellsubstanz, 
Kern  und  Zellteilung:  Leipzig,  1882.  —  Id.,  '87.  Neue  Beitrage  zur  Kenntnis  der 
Zelle  :  A.  m.  A.,  XXIX.  —  Id,,  '88,  Weitere  Beobachtungen  iiber  die  Entwickelung 
der  Spermatosomen  bei  Salamandra  maculosa:  Lbid.,  XXXI.  —  Id., '91-4,  Zelle, 
I.-IV. :  Ergebn.  Anat.  u:  Entivicklimgsgesch.  {Merkel  and  Bonnet),  1891-94.  —  Id., 
'91,  1.     Attraktionsspharen  u.  Centralkorper  in  Gewebs- u.  Wanderzellen :  A.  A. 

—  Id.,  '91,  2.     Neue  Beitrage  zur  Kenntnis  der  Zelle,  II.  Teil :  A.  in.  A.,  XXXVIL 

—  Id., '95,  1.  Uber  die  Struktur  der  Spinalganghenzellen  :  Verhandl.  der  anat. 
Gesellschaft  in  Basel,  17  April,  1895,  p.  19.  —  Id.,  '95,  2.  Zur  Mechanik  der  Zell- 
theilung: A.  m.  A.,  XLVL  — Floderus,  M.,  '96.  Uber  die  Bildung  der  Follikel- 
hlillen  bei  den  Ascidien  :  Z.  w.  Z.,  LXL,  2.  — Fol,  H.,  '73.  Die  erste  Entwickelung 
des  Geryonideies  :  J.  Z.,  VII. — Id.,  '75.  Etudes  sur  le  developpement  des  Mol- 
lusques. — Id.,  '77.  Sur  le  commencement  de  I'henogenie  chez  divers  animaux: 
Arch.  Sci.  Nat.  et  Phys.  Geneve,  LVIII.  See  also  Arch.  Zool.  Exp.,  VL  — Id., 
^79.  Recherches  sur  la  fecondation  et  la  commencement  de  I'henogenie;  Mem.  de 
la  Soc.  de  physique  et  d'hist.  nat.,  GeiUve,  XXVL  — Id.,  '91.  Le  Quadrille  des 
Centres.  Un  episode  nouveau  dans  I'histoire  de  la  fdcondation :  Arch,  des  sci.  phys. 
et  nat.,  15  Avril,  1891  ;  also,  ^.  A.,  9-10,  1891.  —  Foot,  K.,  '94.  Preliminary 
Note  on  the  Maturation  and  Fertilization  of  Allolobophora :  /.  M.,  IX.,  3,  '94. — 
Id.,  '96.  Yolk-nucleus  and  Polar  Rings:  Ibid.,XW.,  i.— Frenzel,  J.,  '93.  Die 
Mitteldarmdrlise  des  Flusskrebses   und   die   amitotische   Zelltheilung:   A.  in.  A., 


348  GENERAL   LITERATURE-LIST 

XLI.  —  Fromman,  C,  '65.  ttber  die  Struktur  der  Bindesubstanzzellen  des 
Ruckenmarks:  CentrL  f.  med.  IVi'ss.,  III.,  6.  —  Id.,  '75.  Zur  Lehre  von  der 
Structur  der  Zellen  :  y.  Z.,  IX.  (earlier  papers  cited).  — Id.,  '84.  Untersuchungen 
liber  Struktur,  Lebenserscheinungen  und  Reactionen  thierischer  und  pflanzlicher 
Zellen  :y.Z.,  XVII. 

GALEOTTI,  GINO,  '93.  i'ber  experimentelle  Erzeugung  von  Unregelmassig- 
keiten  des  karyokinetischen  Processes:  Be/,  zur patholog.  Aiiat.  it.  z.  Allg.  PathoL, 
XIV.,  2,  Jena,  Fischer^  1893.  —  Gardiner,  "W.,  '83.  Continuity  of  Protoplasm  in 
Vegetable  Cells:  PhiL  Trans.,  CLXXIV.  —  Garnault,  '88, '89.  Sur  les  pheno- 
m^nes  de  la  fdcondation  chez  Helix  aspera  et  Arion  empiricorum  :  ZooL  Anz.,  XI., 
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Beitrage  zur  naheren  Kenntniss  der  Schwimmpolypen  :  Z.  w.  Z.,  V.  —  Van 
Gehuchten,  A..  '90.  Recherches  histologiques  sur  I'appareil  digestif  de  la  larve  de 
la  Ptychoptera  contaminata :  La  Cellule,  VI.  —  Giard,  A.,  '77.  Sur  la  significa- 
tion morphologique  des  globules  polaires  :  Revue  sdenli/ique,  XX.  — Id.,  '90.  Sur 
les  globules  polaires  et  les  homologues  de  ces  elements  chez  les  infusoires  cilie's : 
Bulletin  scie7itifique  de  la  France  et  de  la  Belgique,  XXII.  —  Grobben,  C,  '78, 
Beitrage  zur  Kenntniss  der  mannlichen  Geschlechtsorgane  der  Dekapoden  :  Arb. 
ZooL  Ifist.  Wten,\.  —  Gruber,  A.,  '84.  Uber  Kern  und  Kerntheilung  bei  den 
Protozoen :  Z.  iu.  Z.,  XL.  —  Id.,  85.  Uber  klinstliche  Teilung  bei  Infusorien: 
B.  C,  IV.,  23;  v.,  5.  —  Id.,  '86.  Beitrage  zur  Kenntniss  der  Physiologic  und 
Biologic  der  Protozoen  :  Ber.  Naturf.  Ges.  Freiburg.,  I.  — Id.,  '93.  Mikroscopische 
Vivisektion :  Ber.  d.  Naturf.  Ges.  zu  Freiburg.,  VII.,  i  —  Guignard,  L.,  '89. 
Ddveloppement  et  constitution  des  Antherozoides  :  Rev.  gen.  Bat..,  I.  — Id.,  '91,  1. 
Nouvelles  etudes  sur  la  fecondation  :  Ann.  d.  Sciences  Nat.  Bot.,  XIV. —  Id.,  '91,  2. 
Sur  I'existence  des  "  spheres  attractives  "  dans  les  cellules  vegetales  :    C  /?.,  9  Mars. 

HABBRLANDT,  G.,  '87.  Uber  die  Beziehungen  zvvischen  Funktion  und  Lage 
des  Zellkerns:  Fischer.,  1887.  —  Hackel,  E.,  '66.  Generelle  Morphologic. — 
Id.,  '91.  Anthropogenic,  4th  ed.,  Leipzig.,  1891.  —  Hacker,  V.,  '92,  1.  Die 
Furchung  des  Eies  von  .^^iquorea  Forskalea :  A.  in.  A..,  XL. — Id.,  '92,  2,  Die 
Eibildung  bei  Cyclops  und  Camptocanthus  :  Zool.  Jahrb.,  V.  —  Id.,  '92,  3.  Die 
heterotypische  Kerntheilung  im  Cyckis  der  generativen  Zellen  :  Ber.  naturf.  Ges. 
Freiburg,  VI. — Id., '93.  Das  Keimblaschen,  seine  Elemente  und  Lageverander- 
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der  Eireifung :  A.  m.  A..,  XLV.,  2.  —  Id.,  '95,  2.  Zur  Frage  nach  dem  Vorkommen 
der  Schein-Rcduktion  bei  den  Pflanzen  :  Ibid..,  XLVI.  Also  Ann.  Bot.,  IX. — 
Id.,  '95,  3.  Uber  die  Selbstandigkeit  der  vaterlichen  und  miitterlichen  Kernsbe- 
standtheile  wahrend  der  Embryonalentwicklung  von  Cyclops  :  A.  in.  A.,  XLVI.,  4.  — 
Hallez,  P.,  '86.  Sur  la  loi  de  I'orientation  de  I'embryon  chez  les  insectes :  C.  R., 
103,  1886.  — Halliburton,  W.  D.,  '91.  A  Text-book  of  Chemical  Physiology  and 
Pathology  :  London.  —  Id.,  '93.  The  Chemical  Physiology  of  the  Cell :  {Gould- 
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primaren  Zusammenhang  zwischen  den  Furchungszellen  des  Seeigeleies :  A.  ni.  A.., 
XLVIL,  I.  — Hammarsten,  O.,  '94.  Zur  Kenntniss  der  Nucleo-proteids :  Zeit. 
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Wiesbaden,  1895.  —  Hansemann,  D.,  '91.  Karyokinese  und  Cellularpathologie : 
Berl.  Klin.  Woc/ienschrift,  No.  42.  — Id.,  '93.  Spezificitat,  Altruismus  und  die 
Anaplasie  der  Zellen:  Berlin,  1893.  — Hanstein,  J.,  '80.  Das  Protoplasma  als 
Trager  der  pflanzlichen  und  thierischen  Lebensverrichtungen .  Heidelberg.  — 
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GENERAL  LITERATURE-LIST  349 

in  Sydenham  Soc,  X.,  1847.  — Ilartog,  M.  M.,  '91.  Some  Problems  of  Reproduc- 
tion, etc:  Q.  y.,  XXXIII.—  Hatschek,  B.,  '87.  Uber  die  Bedeutung  der 
geschlechtlichen  Fortpflanzung :  Prager  Med.  IVochenschrift,  XLVI.  —  Id.,  '88. 
Lehrbuch  der  Zoologie. — Heidenhain,  M.,  '93.  Uber  Kern  und  Protoplasma: 
Festchr.  2.  zo-ydhr.  Doctorjub.  von  v.  K'dlliker :  Leipzig. — Id.,  '94.  Neue  Unter- 
suchungen  liber  die  Centralkorper  und  ihre  Beziehungen  zum  Kern  und  Zellen- 
protoplasma:  A.  jn.  A.,  XLIII.  —  Id.,  '95.  Cytomechanische  Studien  :  A.  E?itm., 
I.,  4.  —  Heitzmann,  J.,  '73.  Untersuchungen  liber  das  Protoplasma:  Sitzb.  d.  k. 
Acad.  IViss.  ll'ien,  LXVII.  —  Id.,  '83.  Mikroscopische  Morphologie  des  Thier- 
korpers  im  gesunden  und  kranken  Zustande  :  IVien,  1883.  —  Henking,  H.  Unter- 
suchungen liber  die  ersten  Entwicklungsvorgange  in  den  Eiern  der  Insekten,  I., 
II..  III.:  Z.  w.  Z.,  XLIX.,  LI.,  LIV.,  1 890-1 892.  — Henle,  J.,  '41.  Allgemeine 
Anatomic:  Leipzig.  —  Henneguy,  L.  F.,  '91.  Nouvelles  recherches  sur  la  divi- 
sion cellulaire  indirecte :  yourn.  Anat.  et  Physiol.^  XXVII. — Id.,  '93.  Le  Corps 
vitellin  de  Balbiani  dans  I'oeuf  des  Vertebres :  Ibid.,  XXIX. —  Id.,  '96.  Lemons 
sur  la  cellule:  Paris.  —  Hensen,  V., '81.  Physiologic  der  Zeugung :  Her7nann''s 
Physiologie,  VI.  —  Herbst,  C.  Experimentelle  Untersuchungen  liber  den  Ein- 
fluss  der  veranderten  chemischen  Zusammensetzung  des  umgebenden  Mediums 
auf  die  Entvvicklung  der  Thiere,  I.:  Z.  w.  Z.,  LV.,  1892;  II.,  MiU.  Zoo/.  St. 
Neapel,  XI.,  1893;  III.-VI.,  Arch.  Entm.,  II.,  4,  1896.  — Id.,  '94,  '95.  Uber 
die  Bedeutung  der  Reizphysiologie  flir  die  Kausale  Auffassung  von  Vorgangen 
in  der  tierischen  Ontogenese:  Biol.  Centralb.,  XIV.,  XV.,  1894,  1895.  —  Herla, 
v.,  '93.  Etude  des  variations  de  la  mitose  chez  I'ascaride  megalocephale  :  A.  B., 
XIII.  —  Herlitzka,  A.,  '95.  Contributo  alio  studio  della  capacita  evolutiva  dei 
due  primi  blastomeri  nell'  uove  di  Tritone  :  A.  Entm.,  II.,  3.  —  Hermann,  F.,  '89. 
Beitrage  zur  Histologic  des  Hodens :  A.  in.  A.,  XXXIV. — Id.,  '91.  Beitrag  zur 
Lehre  von  der  Entstehung  der  karyokinetischen  Spindel :  Ibid..,  XXXVII. — 
Id.,  '92.  Urogenitalsystem,  Struktur  und  Histiogenese  der  Spermatozoen  :  Merkel 
und  Bonnefs  Ergebnisse,  II.  —  Hertwig,  O.,  '75.  Beitrage  zur  Kenntnis  der 
Bildung,  Befruchtung  und  Teilung  des  tierischen  Eies,  I.:  M.  y.,  I.  —  Id.,  '77. 
Beitrage.  etc.,  II.  :  Ibid.,  III.  — Id.,  '78.  Beitrage,  etc..  III. :  Ibid.,  IV.  —Id.,  '84. 
Das  Problem  der  Befruchtung  und  der  Isotropic  des  Eies,  cine  Theorie  der  Verer- 
bung :  y.  Z.,  XVIII.  —  Id.,  '90,  1.  Vergleich  der  Ei-  und  Samenbildung  bei 
Nematoden.  Eine  Grundlage  flir  cellulare  Streitfragen :  A.  m.  A.,  XXXVI. — 
Id  ,  '90,  2.  Experimentelle  Studien  am  tierischen  Ei  vor,  wahrend  und  nach  der 
Befruchtung:  y.  Z.,  1890. — Id.,  '92,  1.  Urmund  und  Spina  Bifida:  A.  m.  A., 
XXXIX.  —  Id.,  '92,  2.  Aeltere  und  neuere  Entwicklungs-theorieen  :  Berlin. — 
Id  ,  '93.  1.  liber  den  Werth  der  ersten  Furchungszellen  flir  die  Organbildung  des 
Embryo  :  A.  m.  A.,  XLII.  — Id..  93,2.  Die  Zelle  und  die  Gewebe  :  Fischer,  yena, 
1893.  — Id.,  '94.  Zeit  und  Streitfragen  der  Biologic:  Berlin. — Hertwig,  O.  and 
R.,  '86.  Experimentelle  Untersuchungen  liber  die  Bedingungen  der  Bastardbe- 
fruchtung :  y.  Z,  XIX. — Id.,  '87.  t^ber  den  Befruchtungs-  und  Teilungsvorgang 
des  tierischen  Eies  unter  dem  Einfluss  ausserer  Agentien  :  Ibid.,  XX.  —  Hertwig, 
R.,  '77.  liber  den  Bau  und  die  Entwicklung  der  Spirochona  gemmipara :  Ibid., 
XI. — Id.,  '81.  Die  Kerntheilung  bei  Actinosphaerium  Eichhorni :  Ibid.,  XVII. — 
Id.,  '88.  iJber  Kernstruktur  und  ihre  Bedeutung  flir  Zellteilung  und  Befruchtung: 
Ibid.,  IV.,  1888.  —  Id.,  '89.  liber  die  Konjugation  der  Infusorien:  Abh.  der  bayr. 
Akad.  d.  IViss.,  II.,  CI,  XVII.— Id.,  '92.  Uber  Befruchtung  und  Conjugation: 
Verh.  denlsch.  Zool.  Ges.,  Berlin.  —  Id.,  '95.  Uber  Centrosoma  und  Centralspindel : 
Sitz.-Ber.  Ges.  Morph.  und  Phys.,  Miinchen,  1895,  Heft  I.  — Heuser,  E.,  '84. 
Beobachtung  liber  Zelltheilung :  Boi.  Cent. — Hill,  M.  D.,  '95.  Notes  on  the 
Fecundation  of  the  Egg  of  Sphcerechinus  gratiularis  and  on  the  Maturation  and 
■Fertilization  of  the  Egg  oi  Phalltisia  inamtnillata  :  Q.  y,  XXXVIII. —  His.  W., 


350  GENERAL   LITERATURE-LIST 

'74. — Uiisere  Korperform  und  das  physiologische  Problem  ihrer  Entstehung: 
Leipzig.  —  Hofer,  B.,  '89.  Experimentelle  Untersuchungen  liber  den  Einfluss  des 
Kerns  auf  das  Protoplasma  :  J.  Z.,  XXIV.  —  Hofmeister,  '67.  Die  Lelire  von  der 
Pflanzenzelle  :  Leipzig^  1867.  —  Holl,  M.,  '90.  LTber  die  Reifung  der  Eizelle  des 
WM\\Xi^:  Sit  zb.  Acad.  IViss.  IVien.,  XCIX.,  3.— Hooke,  Robt.,  1665.  Mikro- 
graphia,  or  some  physiological  Descriptions  of  minute  Bodies  by  magnifying  Glasses  : 
London.  — Hoyer.  H.,  '90.  tJber  ein  fUr  das  Studiiim  der  "-direkten"  Zelltheilung 
vorzuglich  geeignetes  Objekt :  A.  A.,  V.  —  Humphrey,  J.  E.,  '94.  Nucleolen  und 
Centrosomen  :  Ber.  deiitschefi  bat.  Ges.,  XII.,  5. — Id.,  '95.  On  some  Constituents 
of  the  Cell:  Ann.  Bot.,  IX. —Huxley,  T.  H.,  '53.  Review  of  the  Cell-theory: 
Brit,  and  Foreign  Med.-Chir.  Review.,  XII. — Id., '78.  Evolution  in  Biology, 
Enc.  Brit.,  9th  ed.,  1878 ;  Science  and  Culture^  N.  Y.,  1882. 

ISHIKAWA,  M.,  '91.  Vorlaufige  Mitteilungen  iiber  die  Konjugations- 
erscheinungen  bei  den  Noctiluceen :  Z.  A.y  No.  353,  1891. — Id.,  '94.  Studies  on 
Reproductive  Elements  :  II.,  Noctiluca  tniliaris,  Sur.,  its  Division  and  Spore-forma- 
tion;  Joiirn.  College  of  Sc.  Imp.  Univ.  Japan.,  VI. 

JENSEN,  O.  S.,  '83.  Recherches  sur  la  spermatogenese :  A.  B.,  IV. — 
Johnson,  H.  P.,  '92.  Amitosis  in  the  embryonal  envelopes  of  the  Scorpion:  Bull. 
Mus.  Comp.  Zo'dl,  XXII.,  3.  — Jordan,  E.  O.,  '93.  The  Habits  and  Development 
of  the  Newt:  J.  M.,  VIII.,  2.  — Jordan  and  Eycleshymer,  '94.  On  the  Cleav- 
age of  Amphibian  Ova:  y.  Af.,  IX.,  3,  1894.  —  Julin,  J.,  '93,  1.  Structure  et 
ddveloppement  des  glandes  sexuelles,  ovogenese,  spermatogenese  et  fecondation 
chez  Styleopsis  grossularia:  Bull.  Sc.  de  France  et  de  Belgique.,  XXIV.  —  Id., 
'93.  2.  Le  corps  vitellin  de  Balbiani  et  les  elements  des  Metazoaires  qui  corre- 
spondent au  Macronucleus  des  Infusoires  cilies :  Bull.  Sc.  de  France  et  de 
Belgique,  XXIV. 

KEUTEN,  J.,  '95.  Die  Kerntheilung  von  Englena  viridis  Ehr:  Z.  w.  Z.,  LX. 
—  Kienitz-Gerloff,  F.,  '91.  Review  and  Bibliography  of  Researches  on  Proto- 
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Keimungvon  Closterium  und  Cosmarium  :  Jahrb.f.  wiss.  Bot.,XXll.  —  Klebs,  G., 
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GENERAL   LITERATURE-LISl'  35  I 

C,  '75.  tJber  Differenzierung  des  Protoplasma  an  den  Zellen  thierischer  Gewebe : 
Schr.  natur.  Ver.  Schles.-HolsL,  I.,  3.  —  Id.,  '90.  Die  Entwicklung  von  Petromy- 
zon  Planeri :  A.  m.  A.,  XXXV. 

LAMEBRE,  A.,  '90.  Recherches  sur  la  reduction  karyogamique  :  Bruxelles. — 
Lauterborn,  R.,  '93.  liber  Ban  und  Kerntheilung  der  Diatomeen :  Verh.  d. 
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George,  "65.  Ueber  die  Genese  der  Samenkorper :  A.  m.  A..,  I.  —  Id.,  '67.  Uber 
die  Genese  der  Samenkorper,  II.,  (Terminology)  :  A.  m.  A..,  III.  —  Id.,  '76.  —  Die 
Spermatogenese  bei  den  Amphibien  :  Ibid..,  XII.  —  Id.,  '78.  Die  Spermatogenese  bei 
den  Saugethieren  und  dem  Menschen  :  Ibid..,  XV.  —  Id.  Spermatologische  Beitrage, 
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Id.,  '89.  Beitrage  zur  Kenntniss  des  thierischen  Eies  im  unbefruchteten  Zustande  : 
Spens^eVs  Jahrb.  Anat.  Ont.,  III.  —  Lilienfeld,  L.,  '92,  '93.  tJber  die  Verwand- 
schaft  der  Zellelemente  zu  gewissen  Farbstoffen :  Verh.  Pliys.  Ges.,  Berlin,  1892-3. 
—  Id.,  '93.  Uber  die  Walilverwandtschaft  der  Zellelemente  zu  Farbstoffen : 
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X.  —  Id.,  '96.  On  the  Limit  of  Size  in  the  Regeneration  of  Stentor:  L^ept.  Am. 
MorpJi.  Soc.  Science.,  III.,  Jan.  10,  1896.  —  Loeb,  J.,  '91-2.  Untersuchungen  zur 
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of  physiological  Morphology  :  Wood''s  Holl  Biol.  Lectures,  1893.  —  Id.,  '94;  tJber 
die  Grenzen  der  Theilbarkeit  der  Eisubstanz :  A.  ges.  P.,  LIX.,  6.,  7.  — Lowit,  M., 
'91.  liber  amitotische  Kerntheilung:  B.  C,  XI. — Lukjanow,  '91.  Grundzuge 
einer  allgemeinen  Pathologic  der  Zelle:  Leipzig. — Lustig  and  Galeotti,  '93. 
Cytologische  Studien  liber  pathologische  menschliche  Gewebe  :  Beitr.  Path.  Anat., 
XIV. 

MACALLUM,  A.  B.,  '91.  Contribution  to  the  Morphology  and  Physiology  of 
the  Cell :  Trans.  Canad.  Lnst.,  I.,  2.  — McMurrich,  J.  P.,  '86.  A  Contribution  to 
the  Embryology  of  the  Prosobranch  Gasteropods  :  Studies  Biol.  Lab.  Johns  Hopkins 
Univ.,  III. — Id.,  '95.  Embryology  of  the  Isopod  Crustacea:  /.  M.,  XI.,  i. — 
Maggi,  L.,  '78.  I  plastiduli  nei  ciliati  ed  i  plastiduli  liberamente  viventi :  Atti.  Soc. 
Ital.  Sc.  Nat.  Milano,  XXI.  (also  later  papers).  —  Malfatti,  H.,  '91.  —  Beitrage  zur 
Kenntniss  der  Nucleine  :  Zeit.  Phys.  Chem.,  XVI.  —  Mark,  E.  L.,  '81.  Maturation, 
Fecundation  and  Segmentation  of  Limax  campestris  :  Bull.  Mus.  Coinp.  Zo'ol.  Har- 
vard College,  VI.  —  Maupas,  M.,  '88.  Recherches  experimentales  sur  la  multipli- 
cation des  Infusoires  cilies :  Arch.  Zool.  Exp.,  2me  s^rie,  VI.  —  Id.,  '89.  Le 
rejeunissement  karyogamique  chez  les  Cilies:  Ibid.,  2me  s^rie,  VII. — Id.,  '91. 
Sur  le  determinisme  de  la  sexualite  chez  I'Hydatina  senta:  C.  R.,  Paris. — Mead, 
A.  D..  '95.  Some  observations  on  maturation  and  fecundation  in  Chaetopterus 
pergamentaceus  Cuv. :  J  M.,  X.,  i .  —  Merkel,  F..  '71.  Die  Stlitzzellen  des  mensch- 
iichen  Hodens  :  APi'dler's  Arch.  —  Mertens,  H.,  '93.  Recherches  sur  la  signifi- 
cation du  corps  vitellin  de  Balbiani  dans  Tovule  des  Mammiferes  et  des  Oiseaux  : 
A.  B.,  XIII. — Metschnikoff.  E.,  '66.  Embryologische  Studien  an  Insecten: 
Z.  w.  Z.,  XVI.  — Meves,  F.,  '91.     Uber  amitotische  Kernteilung  in  den  Sperma- 


352 


GENERAL   LITERA  TURE-LIST 


togonien  des  Salamanders,  und  das  Verhalten  der  Attraktionsspharen  bei  derselben : 
A.  A.,  1891,  No.  22.  —  Id.,  '94.  tJber  eine  Metamorphose  der  Attraktionssphare 
;in  den  Spermatogonien  von  Salamandra  maculosa:  A.  m.  A.,  XLIV. — Id., '95. 
tlber  die  Zellen  des  Sesambeines  der  Achillessehne  des  Frosches  (J\!ana  tem- 
poraria)  und  uber  ihre  Centralkorper :  Ibid.,  XLV.  — Meyer,  O.,  '95.  Cellulare 
Untersuchungen  an  Nematodeneiern :  J.  Z.,  XXIX.  (XXII).  —  Mikosch,  '94. 
Uber  Struktur  im  pflanzlichen  Protoplasma :  Verhandl.  d.  Ges.  deutscher  Natiirf. 
und  A'rzte,  1894;  Abteil  f.  Pflanzenphysiologie  u.  Pflanzenaiiatoinie. — Minot, 
C.  S., '77.  Recent  Investigations  of  Embryologists  :  Proc.  Host.  Soc.  Nat.  Hist., 
XIX.  — Id.,  '79.  Growth  as  a  Function  of  Cells:  Ibid.,  XX.  — Id.,  '82.  Theorie 
■der  Genoblasten:  B.  C,  II.,  12.  See  also  A/m.  Nat.,  February,  1880.  — Id.,  '87. 
Theorie  der  Genoblasten:  B.  C,  II.,  12,  1887;  also,  Proc.  Post.  Soc.  Nat.  Hist., 
.XIX.,  1877.  —  Id.,  '91.  Senescence  and  Rejuvenation  : /(!77/r/2.  P/iys.,X\\.,  2. — 
Id.,  '92.  Human  Embryology:  New  York. — von  Mohl,  Hugo,  '46.  Uber  die 
Saftbewegung  im  Innern  der  Zellen  :  Bot.  Zeitung.  —l>a.oove.  J.  E.  S.,  '93.  Mam- 
malian Spermatogenesis:  ^.  ^.,  VIII. —Id., '95.  On  the  Structural  Changes  in 
the  Reproductive  Cells  during  the  Spermatogenesis  of  Elasmobranchs :  Q-  J., 
XXXVIII.  —  Morgan,  T.  H., '93.  Experimental  Studies  on  Echinoderm  Eggs: 
A.  A.,  IX.,  5,  6.  — Id.,  '95,  1.  Studies  of  the  ''Partial"  Larvae  of  Sphasrechinus  : 
A.  Entm.,  II.,  i.  —  Id.,  ^5,  2.  Experimental  Studies  on  Teleost-eggs :  A.  A., 
X.,  19.  — Id.,  '95,  3.  Half-embryos  and  Whole-embryos  from  one  of  the  first  two 
Blastomeres  of  the  Frog's  Egg:  Ibid.,  X.,  19.  —Id.,  '95,  4.  The  Fertilization  of 
non-nucleated  Fragments  of  Echinoderm-eggs :  Arc/i.  Entni.,  II.,  2. — Id.,  '95,  5. 
The  Formation  of  the  Fish-embryo  :  J.  M.,  X.,  2.  —  Id.,  '96, 1.  On  the  Production 
of  artificial  archoplasmic  Centres:  Rept.  of  the  Am.  Morph.  Soc,  Science,  III., 
January  10,  1896.  —  Id.,  '96,  2.  The  Number  of  Cells  in  Larvae  from  Isolated 
Blastomeres  of  Amphioxus  :  Arch.  Entm.,  III.,  2. — Muller,  E., '96.  Uber  die 
Regeneration  der  Augenlinse  nach  Exstirpation  derselben  bei  Triton:  A.  m.  A., 
XLVII.,  I. 

NAGELI,  C,  '84.  Mechanisch-physiologische  Theorie  der  Abstammungslehre  : 
Miinchen  u.  Leipzig,  1884. — Nageli  und  Schwendener,  '67.  Das  Mikroskop. 
(See  later  editions.)  Leipzig.  —  Newport,  G.  On  the  Impregnation  of  the 
•Ovum  in  the  Amphibia:  Phil.  Trans.,  1851,  1853,  1854.  —  Nussbaum,  M., '80. 
Zur  Differenzierung  des  Geschlechts  im  Tierreich  :  A.  m.  A.,  XVIII.  — Id.,  '84,  1. 
Uber  Spontane  und  Kunstliche  Theilung  von  Infusorien :  Verh.  d.  naturh.  Ver. 
pretiss.  Rhineland^  \'^Z\.  —  Id., '84,  2.  tJber  die  Veranderungen  der  Geschlechts- 
producte  bis  zur  Eifurchung :  A.  m.  A.,  XXlll. — Id., '85. —  Uber  die  Teilbarkeit 
•der  lebendigen  Materie,  I. :  A.  m.  A.,  XXVI.  —  Id.,  '94.  Die  mit  der  Entwickelung 
fortschreitende  Differenzierung  der  Zellen  :  Sitz.-Ber.  d.  niederrhein.  Gesellschaft  f. 
Natur-  u.  Heilkunde,  Bonn,  5  Nov.,  1894;  also  B.  C,  XVI.,  2,  1896. 

OGATA,  '83.  Die  Veranderungen  der  Pancreaszellen  bei  der  Secretion : 
A.  A.  /^.  —  Oppel,  A.,  '92.  Die  Befruchtung  des  Reptilieneies  :  A.  m.  A  ,  XXX IX. 
—  Overton,  C.  E.,  '88.  Uber  den  Conjugationsvorgang  bei  Spirogyra :  Ber. 
dentsch.  Bot.  Ges.,  VI.  —  Id.,  '89.  Beitrag  zur  Kentniss  der  Gattung  Volvox  :  Bot. 
Centrb.,  XXXIX.  —  Id.,  '93.  Uber  die  Reduktion  der  Chromosomen  in  den  Kernen 
der  Pflanzen  :  Vierteljahrschr.  naturf.  Ges.  Zurich,  XXXVIII.  Also  Ann.  Bot., 
VII.,  25. 

PALADINO,  G.,  '90.  I  ponti  intercellulari  tra  1'  uovo  ovarico  e  le  cellule  folli- 
colari,  etc:  A.  A.,  V.  —  Palla,  '90.  Beobachtungen  liber  Zellhautbildung  an 
des  Zellkerns  beraubten   Protoplasten.     Elora,   1890.  —  Pfitzner,  W.,  '82.     Uber 


GENERAL   LITERATURE-LIST  353 

den  feineren  Bau  der  bei  der  Zelltheilung  auflfretenden  fadenformigen  Differenzier- 
ungen  des  Zellkerns :  M.  J.,  VII.  —  Id.,  '83.  Beitraige  zur  Lehre  vom  Baue  des 
Zellkerns  und  seinen  Theilungserscheinungen  :  A.  in.  A.,  XXII.  — Pfliiger,  E.,  '83, 
Uber  den  Einfluss  der  Schvverkraft  auf  die  Theilung  der  Zellen :  I.,  Arch.  ges. 
Phys.,XXK\.',  II.,  Ibid.,  XXXII.;  abstract  in  Biol.  Ce?itb.,  III.,  1884.  — Id., '84. 
tJber  die  Einwirkung  der  Schvverkraft  und  anderer  Bedingungen  auf  die  Richtung 
der  Zelltlieilung  :  Arch.  ges.  Phys.,  XXXIV.  — Id.,  '89.  Die  allgemeinen  Lebenser- 
scheinungen  :  Bonn.  —  Platner,  G.,  '86.  Uber  die  Befruchtung  von  Arion  empiri- 
coriim:  A.  m.  A.,  XXXVII.  — Id.,  '89,1.  tJber  die  Bedeutung  der  Richtungs- 
korperchen :  B.  C,  VIII. — Id.,  '89,  2.  Beitrage  zur  Kenntniss  der  Zelle  und 
ihrer  Teilungserscheinungen,  I.-VI. :  A.  m.  A.,  XXXIII.  —  Poirault  and  Raci- 
borski,  '96.  Uber  konjugate  Kerne  und  die  konjugate  Kerntheilung :  B.  C,  XVI., 
I.  —  Prenant, '94.  Sur  le  corpuscule  central:  Bnll.  Soc.  Sci.,  Nancy,  1894. — 
Preusse,  F.,  '95.  Uber  die  amitotische  Kerntheilung  in  den  Ovarien  der  Hemi- 
pteren  :  Z.  w.  Z.,  LIX.,  2.  —  Provost  and  Dumas,  '24.  Nouvelle  thdorie  de  la 
generation:  Ann.  Sci.  Nat.,  \.,  II.  —  Pringsheim,  N., '55.  Uber  die  Befruchtung 
der  Algen:  Monatsb.  Berl.  Akad.,  1855-6.  —  Purkyne  :  Jahrb.  f.  iviss.  Kritik, 
1840. 

RABL,  C,  '85.  Uber  Zellteilung:  M.  J.,  X.  — Id.,  '89,  1.  Uber  Zelltheil- 
ung: A.  A.,  IV. — Id.,  '89,  2.  Uber  die  Prinzipien  der  Histologie:  Verh.  Anat. 
Ges.,  III.  — vom  Rath,  O.,  '91.  Uber  die  Bedeutung  der  amitotischen  Kernteilung 
im  Hoden  :  Zo'dl.  Anz.,  XIV.  —  Id.,  '92.  Zur  Kenntniss  der  Spermatogenese  von 
Gryllotalpa  vulgaris:  A.  in.  A.,  XL.  —  Id., '93.  Beitrage  zur  Spermatogenese 
von  Salamandra  :  Z.  w.  Z.,  LVII. — Id.,  '94.  Uber  die  Konstanz  der  Chromo- 
somenzahl  bei  Tieren :  B.  C,  XIV.,  13.  — Id.,  '95,  1.  Neue  Beitrage  zur  Frage 
der  Chromatinreduction  in  der  Samen-  und  Eireife  :  A.  m.  A.,  XLVI.  —  Id.,  '95,  2. 
Uber  den  feineren  Bau  der  Driisenzellen  des  Kopfes  von  Anilocra,  etc. :  Z.  w.  Z.^ 
LX.,  I.— Rauber,  A.,  '83.  Neue  Grundlegungen  zur  Kentniss  der  Zelle:  M.  J., 
VIII. — Rawitz,  B,,  '95.  Centrosoma  und  Attraktionsphare  in  der  ruhenden  Zelle 
des  Salamanderhodens  :  A.  in.  A.,  XLIV.,  4. — Reinke,  Fr.  '94.  Zellstudien. 
\.,A.  :n.  A.,  XLIII. ;  II.,  Ibid.,  XLIV..  1894.  — Id.,  '95.  Untersuchungen  uber 
Befruchtung  und  Furchung  des  Eies  der  Echinodermen :  Sitz.-Ber.  Akad.  d.  IViss. 
Berlin,  1895,  June  20.  —  Reinke  and  Rode-wald,  '81.  Studien  liber  das  Proto- 
plasma :  Untersuch.  ans  d.  bot.  Inst.  Gottingen.,  II. — Remak,  R.,  '41.  Uber 
Theilung  rother  Blutzellen  beim  Embryo:  Med.  Ver.  Zeit.,  1841.  —  Id.,  '50-5., 
Untersuchungen  iiber  die  .  Entwicklung  der  Wirbelthiere  :  Berlin,  1850-55.  —  Id., 
'58.  iiber  die  Theilung  der  Blutzellen  beim  Embryo:  Mullers  Arch.,  1858. — 
Retzius,  G.,  '89.  Die  Intercellularbriicken  des  Eierstockeies  und  der  Follikelzellen : 
Verh.  Anat.  Ges.,  1889.  —  Rhumbler,  L.,  '93.  Uber  Entstehung  und  Bedeutung 
der  in  den  Kernen  vieler  Protozoen  und  im  Keimblaschen  von  Metazoen  vorkom- 
menden  Binnenkorper  (Nucleolen) :  Z.  w.  Z.,  LVI.  — Rompel,  '94.  Kentrochona 
Nebaliae  n.  g.  n.  sp.,  ein  neues  Infusor  aus  der  Familie  der  Spirochoninen.  Zugleich 
ein  Beitrag  zur  Lehre  von  der  Kernteilung  und  dem  Centrosoma:  Z.  w.  Z.,  LVIIL, 
4.  —  Rosen,  92.  Uber  tinctionelle  Unterscheidung  verschiedener  Kernbestand- 
theile  und  der  Sexual-kerne  :  Cohn's  Beitr.  z.  Biol.  d.  Pflanzen,  V.  —  Id.,  '94.  Neueres 
iiber  die  Chromatophilie  der  Zellkerne  :  Schles.  Ges.  v'dterl.  Knit,  1894.  —  Roux, 
"W.,  '83,  1.  liber  die  Bedeutung  der  Kernteilungsfiguren :  Leipzig.  —  Id.,  '83,  2. 
iiber  die  Zeit  der  Bestimmung  der  Hauptrichtungen  des  Froschembryo  :  Leipzig. — 
Id.,  '85,  iiber  die  Bestimmung  der  Hauptrichtungen  des  Froschembryos  im  Ei, 
und  liber  die  erste  Theilung  des  Froscheies  :  Breslauer  artzl.  Zeitg.,  1885.  —  Id., 
'87,  Bestimmung  der  medianebene  des  Froschembryo  durch  die  Kopulationsricht- 
ung  des  Eikernes  und  des  Spermakernes  :  A.  in.  A.,  XXIX.  —  Id., '88.     Uber  das 


354  GENERAL   LITERATURE-LISr 

kiinstliche  Hervorbringen  halber  Embryonen  durch  Zerstorung  einer  der  beiden 
ersten  Furchungskugeln,  etc. :  Virchow's  Archiv,  1 14.  —  Id.,  '90.  Die  Entwickel- 
ungsmechanik  der Organismen.  Wien,  1 890.  —  Id.,  '92, 1.  Entwickelungsmechanik  : 
Merkel  and  Bonnet^  tl^'g-i  H.  —  Id.,  '92,  2.  Uber  das  entvvickelungsmechanische 
Vermbgen  jeder  der  beiden  ersten  Furchungszellen  des  Eies :    Verh.  Anat.  Ges., 

VI. Id.,  '93, 1.     liber  Mosaikarbeit  und  neuere  Entwickelungshypothesen  :  An. 

Hefte,  Feb.,  1893.  —  Id.,  '93,  2.  Uber  die  Spezifikation  der  Furchungzellen,  etc.: 
B.  C,  XIII.,  19-22.—  Id.,  '94,  1.  Uber  den  "  Cytotropismus"  der  Furchungszellen 
des  Grasfrosches :  Arch.  Ent?n.,  I.,  i,  2.  —  Id., '94,  2.  Aufgabe  der  Entwickel- 
ungsmechanik. etc:  Arch.  Entni.,   I.,  i.     Trans,  in  Biol.  Eectnres,   Wood's  Holl, 

1894. Ruckert,  J.,  '91.     Zur  Befruchtung  des  Selachiereies  :  A.  A..,  VI.  —  Id., 

'92.  1.  Zur  Entvvicklungsgeschichte  des  Ovarialeies  bei  Selachiern :  A.  A.,  VII. — 
Id.  '92,  2.  Uber  die  Verdoppelung  der  Chromosomen  im  Keimblaschen  des  Se- 
lachiereies:  /d/d.,  VIII.  —  Id.,  '93,  2.  Die  Chromatinreduktion  der  Chromosomen- 
zahl  im  Entwicklungsgang  der  Organismen:  Merkel  and  Bonnet,  Erg.,  III.  —  Id., 
'94.  Zur  Eireifung  bei  Copepoden  :  An.  Hefte.  —  ld..,  '95,1,  Zur  Kenntniss  des 
Befruchtungsvorganges :  Sitsb.  Bayer.  Akad.  IViss.,  XXVI.,  i.  — Id.,  '95,  2.  Zur 
Befruchtung  von  Cyclops  strenuus :  A.  A.,  X.,  22.  —  Id.,  '95,  3.  Uber  das  Selb- 
standigbleiben  der  vaterlichen  und  miiterlichen  Kernsubstanz  wahrend  der  ersten 
Entwicklung  des  befruchteten  Cyclops-Eies  :  A.  m.  A.,  XLV.,  3. — Ruge,  G., '89. 
Vorgange  am  Eifollikel  der  Wirbelthiere  :  M.  J.,  XV.  —  Ryder,  J.  A.,  '83.  —  The 
microscopic  Sexual  Characteristics  of  the  Oyster,  etc. :  Bull.  U.  S.  Fish.  Comm., 
March  14,  1883.     Also,  Ajin.  Mag.  Nat.  Hist.,  XII.,  1883. 

SABATIER,  A.,  '90.  De  la  Spermatogenese  chez  les  Locustides;  Comptes 
Rend.y  CXI.,  '90.  —  Sachs.,  J.,  '82.  Vorlesungen  iiber  Pflanzen-physiologie  ;  Leip- 
zig.—  Id.  liber  die  Anordnung  der  Zellen  in  jiingsten  Pflanzentheile :  Arb.  Bot. 
Inst.  Wurzburg,  II.  —  Id.,  '92.  Physiologische  Notizen,  II.,  Beitrage  zur  Zel- 
lentheorie:  Flora,  1892,  Heft  I.  —  Id., '93.  Stoff  und  Form  der  Pflanzen-organe  ; 
Gesammelte  Abhandlung,  II .,  1 893.  —  Id.,  '95.  Physiologische  Notizen,  IX.,  weitere 
Betrachtungen  iiber  Energiden  und  Zellen:  Flora,  LXXXI.,  2.  —  Sala,  L.,  '95. 
Experimentelle  Untersuchungen  iiber  die  Reifung  und  Befruchtung  der  Eier  bei 
Ascaris  megalocephala ',  A.  in.  A.,  XL.  — Sargant,  Ethel,  '95.  Some  details  of 
the  first  nuclear  Division  in  the  Pollen-mother-cells  of  Liliimt  martagon ;  Joiirn. 
Roy.  Mic.  Soc,  1895,  part  3.  —  Schafer,  E.  A.,  '91.  General  Anatomy  or  Histol- 
ogy: in  Qtiain's  Anatoiny,  I.,  2,  loth  ed.,  London.  —  Schaudinn,  F.,  '95.  Uber 
die  Theilung  von  Amoeba  binucleata  Gruber:  Sitz.-Ber.  Ges.  Natnrforsch.,  Fretinde, 
Berlin,  Jahrg.  1895,  No.  6.  —  Id.,  '96.  Uber  den  Zeugungskreis  von  Parainoeba 
Eilhardi:  Sitz.-Ber.  Ges.  Natnrforsch.,  Freunde,  Berlin,  1896,  Jan.  13.  —  Schewi- 
akoff,  "W., '88.  Uber  die  karyokinetische  Kerntheilung  der  Euglypha  alveolata: 
M.J.,yA\\.  —  Schiefferdecker  and  Kossel,  '91.  Die  Gevvebe  des  Menschlichen 
Korpers :  Braunschweig.  —  Schimper,  '85.  Untersuchungen  liber  die  Chlorophyll- 
korper,  etc. :  Zeitsch.  wiss.  Bot.,  XVI.  —  Schleicher,  W,,  '78.  Die  Knorpelzell- 
theilung.  Ein  Beitrag  zur  Lehre  der  Theilung  von  Gewebezellen  :  Centr.  ined.  Wiss. 
Berlin,  1878.  Also  A.  in.  A.,  XVI,  1879.  — Schleiden,  M-  J-^  '38.  Beitrage  zur 
Phytogenesis :  Miiller's  Archiv,  \^t^%.  [Trans,  in  Sydenham  Soc,  XII.:  London, 
1847.]  — Schloter,  G.,  '94.  Zur  Morphologie  der  Zelle :  A.  in.  A.,  XLIV..  2. — 
Schmitz, '84,  Die  Chromatophoren  der  Algen.  —  Schneider,  A.,  '73.  Unter- 
suchungen iiber  Plathelminthen :  Jahrb.  d.  oberhcss.  Ges.  f.  Natur- Heilkunde, 
XIV.,  Giessen.  —  Schneider,  C,  '91.  Untersuchungen  iiber  die  Zelle:  Arb.-Zool. 
Inst,  li'ien,  IX.,  2.  —  Schottlander,  J.,  '88.  Uber  Kern  und  Zelltheilungsoor- 
gjinge  in  dem  Endothel  der  entziindeten  Hornhaut:  A.  in.  A.,  XXXI.  —  Schultze, 
Max,  '61.     Uber  Muskelkorperchen  und  das  was  man  eine  Zelle  zu  nennen  hat ; 


GENERAL   LITERATURE-LIST  355 

Arch.  Anat.  Fhys.,  1861.  —  Schultze,  O.,  '87.  Untersuchungen  iiber  die  Reifung 
und  Befruchtung  des  Amphibien-eies  :  Z.  iv.  Z.yXLY.  —  Id., '94.  Die  kiinstliche 
Erzeugung  von  Doppelbildungen  bei  Froschiarven,  etc.:  Arch.  Entin..,  I.,  2. — 
Schwann.  Th.,  '39.  Mikroscopische  Untersuchungen  iiber  die  Uebereinstimmung 
in  der  StrucJ;ur  und  dem  Wachsthum  der  Thiere  und  Pflanzen  :  Berlifi.  [Trans,  in 
Sydenham  Soc.^XW.:  Loncioti,  1847.]  —  Schwarz,  Fr., '87.  Die  Morphologische 
und  chemische  Zusammensetzung  des  Protoplasmas :  Breslaii.  —  Schweigger- 
Seidel,  O,,  '65.     Uber  die  Samenkorperchen  und  ihre  Entwickelung :  A.  in.  A.,  I. 

—  Sedgwick,  A., '85-8.  The  Development  of  the  Cape  Species  of  Peripatus, 
I.-VI.:  Q.  7.,  XXV.-XXVIII.  — Id.,  '94.  On  the  Inadequacy  of  the  Cellular 
Theory  of  Development,  etc.:  Q.  J.,  XXXVIL,  i.  — Seeliger,  O.,  '94.  Giebt  es 
geschlechtlicherzeugte  Organismen  ohne  mlitterliche  Eigenschaften  ?  :  A.  Ent.^  I.,  2. 

—  Selenka,  E.,  '83.  Die  Keimblatter  der  Echinodermen  :  Stiidien  uber  Entwick.^ 
II,  Wiesbaden,  1883.  —  Sertoli,  E.,  '65.  Dell'  esistenza  di  particolori  cellule 
ramificate  dei  canaliculi  seminiferl  del  testicolo  umano :  //  Morgagni.  —  Sied- 
lecki,  M.,  '95.  Uber  die  Struktur  und  Kerntheilungsvorgange  bei  den  Leucocy- 
ten  der  Urodelen :  Anz.  Akad.  JVi'ss.,  Krakau,  1895.  —  Sobotta,  J.,  '95.  Die 
Befruchtung  und  Furchung  des  Eies  der  Maus :  A.  in.  A..,  XL.  —  Solger,  B., 
'91.  Die  radiaren  Strukturen  der  Zellkorper  im  Zustand  der  Ruhe  und  bei  der 
Kerntheilung :  Berl.  Klin.  IVochenschr.,  XX.,  1891.  —  Spallanzani,  1786.  Ex- 
periences pour  servir  a  I'histoire  de  la  generation  des  animaux  et  des  plantes : 
Geneva.  —  Strasburger,  E..  '75.  Zellbildung  und  Zelltheilung :  ist  ed.,  Jena, 
1875.  — Id., '77.  Uber  Befruchtung  und  Zelltheilung  :  7.  Z.,  XL  — Id., '80.  Zell- 
bildung und  Zellteilung:  3d  ed.  —  Id., '82.  Uber  den  Theilungsvorgang  der  Zell- 
kerne  und  das  Verhaltniss  der  Kerntheilung  zur  Zelltheilung:  A.  in.  A.,  XXL  — 
Id.,  '84.  1.  Die  Controversen  der  indirecten  Zelltheilung:  Ibid.,  XXIII.  —  Id., 
'84.  2.  Neue  Untersuchungen  iiber  den  Befmchtungsvorgang  bei  den  Phaneroga- 
men,  als  Grundlage  fur  eine  Theorie  der  Zeugung :  Jena,  1884.  —  Id.,  '88.  Uber 
Kern-  und  Zellteilung  im  Pflanzenreich,  nebst  einem  Anhang  iiber  Befruchtung : 
Jena.  —  Id.,  '89.  Uber  das  Wachsthum  vegetabilischer  Zellhaute :  Hist.  Bei., 
II.,  Jena. — Id.,  '91.  Das  Protoplasma  und  die  Reizbarkeit :  Rektoratsrede,  Bonn, 
Oct.  18,  1891.  Jena,  Fischer.— lA., '92.  Histologische  Beitrage,  Heft  IV. :  Das 
Verhalten  des  Pollens  und  die  Befruchtungsvorgange  bei  den  Gymnospermen, 
Schwarmsporen,  pflanzliche  Spermatozoiden  und  das  Wesen  der  Befruchtung : 
Fischer,  Jena,  1892.  —  Id.,  '93.  1.  Uber  die  Wirkungssphare  der  Kerne  und  die 
Zellengrosse  :  Hist.  Beitr.,  V.  —  Id.,  '93,  2.  Zu  dem  jetzigen  Stande  der  Kern-  und 
Zelltheilungsfragan  :  ^.  .4,,  VIIL,  p.  177.  —  Id., '94.  Uber  periodische  Reduktion 
der  Chromosomenzahl  im  Entvvicklungsgang  der  Organismen:  B.  C,  XIV.  —  Id., 
'95.  Karyokinetische  Probleme  :  Jahrb.  f.  wiss.  Botanik,  XXVIIL,  i.  —  Van  der 
Stricht,  O.,  '92.  Contribution  a  I'etude  de  la  sphere  attractive:  A.  B.,  XII.,  4. — 
Id.,  '95,  1.  La  maturation  et  la  fecondation  de  I'oeuf  d'Amphioxus  lanceolatus : 
Bull.  Acad.  Roy.  Belgiqne,  XXX.,  2.  — Id.,  '95,  2.  De  I'origine  de  la  figure  achro- 
matique  de  I'ovule  en  mitose  chez  le  Thysanozoon  Brocchi :  Verhandl.  d.  anat. 
Versaniinl.  in  Strassbiirg,  1895,  p.  223.  —  Id.,  '95,  3.  Contributions  a  I'etude  de 
la  forme,  de  la  structure  et  de  la  division  du  noyau :  Bidl.  Acad.  Roy.  Sc.  Belgiqiie, 
XXIX.  — Strieker,  S.,  '71.  Handbuch  der  Lehre  von  den  Geweben :  Leipzig.— 
Stuhlmann,  Fr.,  '86.  Die  Reifung  des  Arthropodeneies  nach  Beobachtungen  an 
Insekten,  Spinnen,  Myriopoden  und  Peripatus  :  Ber.  Naturf.  Ges.  Freiburg,  I. — 
Swaen  and  Masquelin,  '83.     Etude  sur  la  Spermatogenese  :  A.  B.,  IV. 

THOMA,  R.,  '96.  Text-book  of  General  Pathology  and  Pathological  Anatomy  : 
Tran.s.  by  A.  Bruce,  London.  — Thomson.  Allen.  Article  '•  Generation"  in  Todd's 
Cyclopedia.  —  Id.  Article  ''Ovum"  in  Todd's  Cyclopedia.  —  Tyson,  James. '78. 
The  Cell-doctrine  :  2d  ed.,  Fhiladelphia. 


356  GENERAL   LITERATURE-LIST 

USSOW,  M.,  '81.  Untersuchungen  iiber  die  Entwickelung  der  Cephalopoden  : 
Arch.  Biol..,  II. 

VEJDOVSKY,  F.,  '88.  Entwickelungsgeschichtliche  Untersuchungen,  Heft  I. : 
Reifung,  Befruchtungund  Furchungdes  Rhynchelmis-Eies  :  Frag.,  i888.  —  Verworii, 
M.  '88.  Biologische  Protisten-studien  :  Z.  w.  Z.,  XLVL  — Id.,  '89.  —  Psycho- 
•physiologische  Protisten-studien:  Jena.  —  Id.,  '91.  Die  physiologische  Bedeutung 
des  Zellkerns:  Pfliiger's  Arch.  f.  d.  ges.  Physiol.,  LI. — Id.,  '95.  AUgemeine 
Physiologic:  Jena.  —  Vircho-w,  R., '55.  Cellular-Pathologie  :  Arch.  Path.  Anat. 
Phys.,  VIII.,  I. — Id.,  '58.  Die  Cellularpathologie  in  ihrer  Begriindung  auf  physio- 
logische und  pathologische  Gewebelehre :  Berlin,  1858.  —  De  Vries,  H.,  '89. 
Intracellulare  Pangenesis :  Jena. 

WALDEYER,  "W.,  '70.  Eierstock  und  Ei :  Leipzig.  — Id.,  '87.  Bau  und 
Entwickekmg  der  Samenfaden :  Verh.  d.  Anat.  Leipzig,  1887.  —  I^-?  '^8.  t-ber 
Karyokinese  und  ihre  Beziehungen  zu  den  Befruchtungsvorgangen :  A.  m.  A., 
XXXII.  [Trans,  in  Q.  J.~\  — Id.,  '95.  Die  neueren  Ansichten  iiber  den  Bau  und 
das  Wesen  der  Zelle :  Deutsch.  Med.  Wochenschr .,  No.  43,  ff.,  Oct.  ff.,  1895. — 
"Warneck,  N.  A.,  '50.  Ueber  die  Bildung  und  Entwickelung  des  Embryos  bei 
Gasteropoden :  Bidl.  Soc.  Imp.  Nat.  Moscou,  XXIII.,  i.  — Watas^,  S.,  '91. 
Studies  on  Cephalopods  ;  I.,  Cleavage  of  the  Ovum  :  /. .]/.,  IV.,  3.  —Id.,  '92.  On 
the  Phenomena  of  Sex-differentiation:  Ibid.,  VI.,  2,  1892.  —  Id.,  '93,  1.  On  the 
Nature  of  Cell-organization:  Wood''s  Noll  Biol.  Lectures,  1893.  —  Id.,  '93,  2. 
Homology  of  the  Centrosome :  J.  M.,  VIII.,  2. — Id.,  '94.  Origin  of  the  centro- 
some:  Biological  Lectures,  Wood''s  Holl,  1894.  —  Weismann,  A.,  '83.  Uber 
Vererbung :  Jena. — Id.,  '85.  Die  Kontinuitat  des  Keimplasmas  als  Grundlage 
einer  Theorie  der  Vererbung:  Jena.  —  Id.,  '86,  1.  Richtungskorper  bei  partheno- 
genetischen  Eiern:  Zool.  Anz.,  No.  233. — Id.,  '86,  2,  Die  Bedeutung  der  sexuel- 
len  Fortpflanzung  fiir  die  Selektionstheorie  :  Jena.  —  Id.,  '87.  tJber  die  Zahl  der 
Richtungskorper  und  iiber  ihre  Bedeutung  fiir  die  Vererbung:  Jejia.  —  Id.,  '91,  1. 
Essays  upon  Heredity.  First  Series  :  Oxford.  —  Id., '91,  2.  Amphimixis,  oder  die 
Vermischung  der  Individuen  :  Jena,  Fischer.  —  Id.,  '92.  Essays  upon  Heredity, 
Second  Series  :  Oxford,  1892.  —  Id.,  '93.  The  Germ-plasm  :  New  York.  —  Id.,  '94. 
Aeussere  Einfliisse  als  Entwicklungsreize :  Jena.  —  Wheeler,  W.  M.,  '89.  The 
Embryology  of  Blatta  Germanica  and  Doryphora  decemliiieata :  J.  M.,  III. — 
Id., '93.  A  Contribution  to  Insect-embryology:  Ibid.,  VIII.  i.  — Id., '95.  The 
Behavior  of  the  Centrosomes  in  the  Fertilized  Egg  oi  Myzostoma glabrnni :  Ibid.,  X. 

—  Whitman,  C.  O.,  '78.  The  Embryology  of  Clepsine  :  Q.  J.,  XVIII.  —  Id.,  '87. 
The  Kinetic  Phenomena  of  the  Egg  during  Maturation  and  Fecundation  :  J.  M.,  I..  2. 

—  Id.,  88.  The  Seat  of  Formative  and  Regenerative  Energy:  Ibid.,  II. — Id. ,'93. 
The  Inadequacy  of  the  Cell-theory  of  Development:  IVood's  Holl  Biol.  Lectures, 
1893.  —  Id.,  94.  Evolution  and  Epigenesis  :  Ibid.,  \Zc)^.  —  Wiesner,  J., '92.  Die 
Elementarstruktur  und  das  Wachstum  der  lebenden  Substanz :  Wien. — Wilcox, 
E.  v.,  '95.  Spermatogenesis  of  Caloptenus  and  Cicada:  Bull,  of  the  Museum 
of  Comp.  Zool.,  Llarvard  College,  Vol.  XXVII.,  Nr.  i.  — Will,  L.,  '86.  Die 
Entstehung  des  Eies  von  Colymbetes :  Z.  w.  Z.,  XLIII.  —  Wilson,  Edm.  B., 
'92.  The  Cell-lineage  of  Nereis:  J.  M.,  VI.,  3.-— Id.,  ^3.  Amphioxus  and  the 
Mosaic  Theory  of  Development :  Ibid,  VIII  ,  3.  — Id.,  '94.  The  Mosaic  Theory  of 
Development:  Wood'' s  Holl  Biol.  Led.,  1894. — Id.,  '95,  1.  Atlas  of  Fertilization 
and  Karyokinesis :  New  York,  Macmillan. — Id.,  '95,  2.  Archoplasm,  Centro- 
some, and  Chromatin  in  the  Sea-urchin  Egg:  /.  M.,  XI.  —Id.,  '96.  On  Cleavage 
and  Mosaic-work:  A.  Entm..  III.,  i.  —  Wilson  and  Mathews,  '95.  Maturation, 
Fertilization,  and  Polarity  in  the  Echinoderm  Egg:  /.  M.,  X.,  i.  —  Wolff,  Caspar 


GENERAL   LITERATURE-LIST  357 

Friedrich,  1759.  Theoria  Generationis.  —  Wolff,  Gustav,  '94.  Bemerkungen 
zum  Darwinismus  mit  einem  experimentellen  Beitrag  zur  Physiologic  der  Entwick- 
lung:  B.  C,  XIV.,  17.  — Id.,  '95.  Die  Regeneration  der  Urodelenlinse :  Arch. 
Entm.,  I.,  3. — Wolters,  M.,  '91.  Die  Conjugation  und  Sporenbildung  bei 
Gregarineni  A.m.  A.,  XXXVII. 

ZACHARIAS,  O.,  '85.  Uber  die  amoboiden  Bewegungen  der  Spermatozoen 
von  Polyphemus  pediculus :  Z.  w.  Z.,  XLI. — Zacharias,  E.,  '93,  1.  tJber  die 
chemische  Beschaffenheit  von  Cytoplasma  und  Zellkern  :  Ber.  deutsch.  bot.  Ges., 
II.,  5.  — Id.,  '93,  2.  Uber  Chromatophilie :  Ibid.,  1893.  — Id.,  '95.  Uber  das 
Verhalten  des  Zellkerns  in  wachsenden  Zellen  :  Flora,  81,  1895.  —  Id., '94.  Uber 
Beziehungen  des  Zellenwachstums  zur  Beschaffenheit  des  Zellkerns  :  Berichte  der 
deiUschen  botaii.  Gesellschap,  XII.,  5.  —  Ziegler,  E.,  '88.  Die  neuesten  Arbeiten 
iiber  Vererbung  und  Abstammungslehre  und  ihre  Bedeutung  fiir  die  Pathologie : 
Beitr.  zur  path.  Anal.,  IV. — Id.,  '92.  Lehrbuch  der  allgemeinen  pathologischen 
Anatomie  und  Pathogenese,  7th  ed.  :  Jena.  — Ziegler,  H.  E.,  '87.  Die  Entsteh- 
ung  des  Blutes  bei  Knochenfischenembryonen  :  A.  in.  A.  —  Id.,  *91.  Die  biolo- 
gische  Bedeutung  der  amitotischen  Kerntheilung  im  Tierreich  :  B.  C.  XI. —  Id.,  '94. 
tlber  das  Verhalten  der  Kerne  im  Dotter  der  meroblastischen  Wirbelthiere  :  Ber. 
Natnrf.  Ges.  Freiburg,  1894.  —  Id., '95.  Untersuchungen  liber  die  Zelltheilung: 
Verhajidl.  d.  deutsch.  Zool.  Ges.,  1895.  —  Ziegler  and  vom  Rath.  Die  amitotische 
Kerntheilung  bei  den  Arthropoden  :  B.  C,  XI.  — Zimniermann,  A.,  '93^  1.  Bei- 
trage  zur  Morphologic  und  Physiologic  der  Pflanzenzelle  :  Tiibingen.  —  Zimmer- 
maiin,  K.  W.,  '93.  2.  Studien  iiber  Pigmentzellen,  etc:  A.  m.  A.,  XLI. — 
Zoja,  R.,  '95, 1.  Sullo  sviluppo  dei  blastomcri  isolati  dalle  uova  di  alcune  medusc : 
A.  Entni.,  I.,  4;  II.,  i  ;  II.,  IV.  —  Id.,  '95,  2.  Sulla  independenza  della  cromatina 
patcrna  e  materna  nel  nuclco  delle  cellule  embrionali :  A.  A.,  XI.,  10. 


APPENDIX 


1.  P.  15,  Fig,  6.  According  to  Meves  (A.  m.  A.,  XLVIII.  i,  '96),  the  attraction-sphere 
of  the  resting  spermatogonium  of  the  salamander  contains  two  centrosomes,  and  these  are 
much  smaller  than  the  body  figured  by  Rawitz.  I  find  this  to  be  also  the  case  in  Amphiuma^ 
where  the  attraction-sphere  is  sharply  defined  and  the  centrosomes,  though  very  small,  are 
extremely  distinct. 

2.  P.  16.  Conklin  (^Am.  Nat.^  Jan.,  '97)  states  that  in  cells  of  the  intestinal  epithelium 
of  the  isopod  Porcellio,  the  nuclear  membrane  is  sometimes  absent  on  one  side  of  the 
nucleus,  and  the  linin-network  here  shows  an  unbroken  continuity  with  the  cytoplasmic 
thread-work. 

3.  P.  19.  Many  recent  researches  indicate  that  no  general  formula  can  yet  be  given 
for  protoplasmic  structure.  The  alveolar  theory  has  gained  many  adherents,  yet  the  opinion 
is  gaining  ground  that  in  varying  physiological  states  of  the  cell  protoplasm  may  undergo 
more  extensive  structural  changes  than  was  formerly  supposed.  Flemming  himself  now 
admits  the  existence  of  alveolar  structure  in  protoplasm  (Merkel  u.  Bonnet,  Erg.^  V.,  '96), 
though  his  conception  still  differs  widely  from  Biitschli's.  He  now  recognizes:  (i)  An 
apparently  homogeneous  ground-substance  ;  (2)  In  many  cases  this  substance  is  filled  with 
minute  vacuoles,  giving  the  alveolar  structure  of  BUtschli;  (3)  In  many  kinds  of  ce\\%,Jibres, 
lying  in  the  ground-substance,  and  perhaps  consisting  of  rows  of  microsomes;  (4)  In  some 
cases  separate  granules  embedded  in  the  ground-substance  and  many  of  them  certainly 
preformed  in  life.  Flemming  practically  abandons  his  early  view  that  the  thread-work  or 
reticulum  probably  represents  the  "  living  substance,"  and  now  admits  that  any  or  all  of  the 
four  elements  enumerated  above  may  be  the  substratum  of  vitality. 

4.  P.  27,  1.  I  (see  also  p.  246).  The  large  size  of  the  nuclei  in  embryonic  as  compared 
with  adult  cells  was  noted  by  many  early  observers.  Its  importance  was  emphasized  by 
Sachs  ('82),  and  KoUiker  ('85),  and  later  by  Minot  {Proc.  Am.  Ass.  Adv.  Sci.,  '90),  who 
concludes  that  a  relative  increase  in  the  quantity  of  cytoplasm  is  in  general  characteristic  of 
advancing  age.  Schwarz  (Cohn's  Beitrdge,  IV.),  who  has  studied  the  size-relations  carefully 
in  growing  root-tips,  finds  the  largest  nuclei  not  in  the  actively  dividing  tissue  (meristem), 
but  in  the  slightly  older,  rapidly  growing  cells.  At  a  later  period  the  nuclei  diminish  both 
in  relative  size  and  in  stainirig-capacity  {i.e.  nuclein-content).  Zacharias  ('94,  '95)  reaches 
essentially  the  same  result. 

5.  P.  27,  1.  19.  In  nuclei  of  the  spinning-glands  of  caterpillars  Korschelt  {A.  m.  A., 
XLVII.  3,  '96)  is  able  to  see  the  chromatin-granules  clearly  in  life.  C/.  Meves  {A.  m.  A., 
XLVIII.  '96). 

6.  P.  40,  1.  30.  Heidenhain  and  Cohn  {Morph.  Arb.,  VII.  i,  '97)  figure  the  double 
centrosomes  in  this  position  in  several  forms  of  embryonic  epithelia,  including  the  epidermis, 
the  neural  and  sensory  epithelia,  the  mesenteron,  the  walls  of  the  mesoblastic  somites  and 
those  of  the  Wolffian  duct. 

7.  P.  43.  Arthur  Meyer  {BoL  ZeiL,  XI.,  XII.,  '96)  maintains  that  the  cells  of  all  forms 
of  plant  tissues  are  in  organic  continuity,  and  accepts  the  probability  that  the  bodies  of  all 
plants  and  animals  are  practically  continuous  masses  of  protoplasm. 

358  « 


358^ 


APPENDIX 


8.  P.  53.  Origin  of  the  mitotic  figure.  It  is  not  made  sufficiently  clear,  in  this  brief 
account,  that  the  origin  of  the  achromatic  figure  varies  in  different  cases,  though  this  fact  is 
pointed  out  in  a  number  of  other  places  {cf.  pp.  49,  50)'.  In  all  cases  where  the  centro- 
some  is  extra-nuclear,  the  asters  appear  to  be  of  cytoplasmic  origin  and  arise  through  a 
radial  grouping  of  the  cytoplasmic  network.  The  spindle,  on  the  other  hand,  may  be  en- 
tirely of  intra-nuclear  origin,  being  formed  from  the  linin-network  (in  many  plant  cells  and 
embryonic  animal  cells) ;  or  it  may  have  a  double  origin,  the  central  spindle  being  formed 
outside  the  nucleus,  while  the  mantle-fibres  arise,  in  part  at  least,  from  the  linin-network 
(spermatogonia  of  the  salamander.  Fig.  21,  etc.).  The  difference  presents  no  difficulty  in 
view  of  the  fact  that  the  linin-network  is  no  more  than  the  intra-nuclear  part  of  the  general 
cell-reticulum  (p.  214),  from  which  the  entire  amphiaster  is  formed. 

9.  P.  82,  1.  23.  The  -connection  between  the  attraction-spheres  has  been  shown  by 
Meves  (//.  m.  A.,  XLVIII.  i,  '96)  to  be  the  remains  of  the  central  spindle. 

10.  P.  112,  Foot-note  2.  In  Hydra  many  of  the  ovarian  cells  break  up  and  their  nuclei 
are  ingulfed  by  the  ovum  to  form  the  "pseudo-cells."  (Brauer,  Z.  w.  Z.,  LII.,  '91.)  In 
Tubularia  a  number  of  germ-cells  fuse  to  form  a  syncytium,  in  which  the  nucleus  enlarges 
to  form  the  germinal  vesicle,  while  the  others  degenerate  as  pseudo-cells.  (Doflein,  Z.  w.  Z., 
LXII.  I,  '96.) 

11.  P.  125,  1.  38.  Meves  {Mitth.  f.  d.  Ver.  Schlesw.- Hoist.,  Aerzte,  V.  5,  '97)  has  estab- 
lished the  truth  of  Hermann's  conjecture  by  tracing  the  "  Nebenkorper  "  to  the  attraction- 
sphere  of  the  early  spermatid.  At  an  early  period  it  contains  two  centrosomes  which  are 
said  to  give  rise  respectively  to  the  ring  and  to  the  deeply  staining  sphere.  The  latter  en- 
larges to  form  the  middle-piece;  the  ring  does  not  form  the  "fin,"  as  Hermann  believed, 
but  the  posterior  end  of  the  middle-piece,  and  is  homologized  by  Meves  with  the  "  end- 
knob  "  of  Ballowitz.  The  axial  filament  grows  out  from  the  more  peripherally  placed  cen- 
trosome  which  afterwards  gives  rise  to  the  ring,  and  it  has  no  connection  with  the  nucleus. 

12.  P.  156,  last  line.  Through  a  misapprehension  of  Van  Beneden's  meaning,  I  have 
incorrectly  stated  his  opinion  regarding  the  origin  of  the  centrosomes  (see  Sdence,  V.  105, 
Jan,  I,  '97).  He  did  not  in  fact  commit  himself  to  any  positive  conclusion,  but  somewhat 
doubtfully  expressed  the  opinion  that  both  attraction-spheres,  and  hence  by  implication 
both  centrosomes,  were  probably  derived  from  the  egg,  i.e.  from  the  second  "  pseudo- 
karyokinetic  "  (maturation)  figure. 

13.  P.  159I  Lillie  {Science,  W.  II 4,  March  5, '97)  has  reached  the  remarkable  result 
that  in  the  lamellibranch  Unio  both  egg-  and  sperm-centrosomes  disappear,  the  cleavage- 
centrosomes  being  a  new  formation  of  maternal  origin. 

14.  P.  159,  Foot-note.  For  Hertwig's  full  paper,  which  is  of  great  interest  and  impor- 
tance, see  Festschrift  fUr  Gegenbaur,  Engelmann,  Leipzig,  '96. 

15.  P.  192,  1.  3.  Meves's  new  and  very  thorough  investigations  (^A.  m.  A.,  XLVIII. 
I,  '96)  lend  no  support  to  vom  Rath's  account.  There  are  but  two  generations  of  sperma- 
tocytes and  two  "  maturation-divisions."  The  first  of  these  is  of  the  heterotypical  form,  but 
no  tetrads  are  formed,  and  the  second  division  occurs  by  longitudinal  splitting  of  the  chro- 
mosomes. Both  divisions  are  interpreted  as  "equation-divisions,"  and  there  is  therefore  no 
reduction  in  Weismann's  sense.  All  these  conclusions  are  sustained  by  observations  on 
Amphiuma  made  by  Mr,  J,  H,  McGregor,  in  the  Columlna  lalwratory.  Reduction  in  the 
amphibia  would  therefore  seem  to  take  place  substantially  in  the  same  manner  as  in 
Ascaris. 

16.  P.  192,  summary.  The  origin  and  meaning  of  the  tetrads  still  remains  problem- 
atical, and  some  of  the  recent  work  shows  the  necessity  for  great  caution  in  the  attempt  to 
draw  general  conclusions.  In  a  recent  paper  (//.  m.  A.,  XLVIII.  4,  '97)  von  Klinckowstrom 
describes  and  figures  tetrads  in  the  second  polar  spindle  of  Prosthecemus,  a  Platode.  R.  Hert- 
wig  (^Festschrift  f.  Gegenbaur,  Leipzig,  '96)  has  observed  them  as  an  alinormality  in  the 
i-Zeaz/iZ^^-spindle  of  unfertilized  sea-urchin  eggs  after  poisoning  with  strychnine.     The  first 


APPENDIX 


358r 


of  these  results  is  quite  unintelligible  unless  it  be  pathological,  as  in  the  case  described  by 
Hertwig.  It  may  be  recalled  that  Flemming  discovered  typical  tetrads  in  the  spermato- 
cytes of  the  salamander,  but  regarded  them  as  "anomalies"  (^cf.  p.  192),  and  the  later 
studies  of  Meves  and  McGregor  render  it  probable  that  his  interpretation  was  correct  {cf. 
preceding  note).  These  highly  interesting  observations  probably  open  the  way  to  a  future 
better  understanding  of  the  tetrads;  but  for  the  present  they  leave  the  whole  subject  of 
reduction  in  a  state  of  confusion  worse  confounded. 

17.  P.  194,  line  12.  Meves  {I.e.)  apparently  disproves  the  accounts  both  of  Flemming 
and  of  vom  Rath,  and  shows  that  the  reduced  number  appears  in  but  two  cell-generations, 
as  in  Ascaris. 

18.  P.  197.  Calkins  {Bull.  Torrey  Bot.  Club,  XXIV.  3,  '97)  has  discovered  typical  tetrads 
in  the  spore- formation  of  ferns  {Adiantum,  Pteris).  The  tetrads  arise,  as  in  insects  and 
copepods,  by  one  longitudinal  and  one  transverse  division  of  a  primary  chromatin-rod;  and 
the  facts  indicate  further,  that  the  actual  reduction  is  effected  by  the  second  cell-division. 
It  is  an  interesting  fact  that  the  tetrads  vary  in  mode  of  origin,  some  of  them  forming  rings, 
others  arising  from  completely  split  rods,  which  afterwards  divide  transversely. 

19.  P.  199,  1.  29.  In  a  highly  interesting  paper  {Jahrb.  wiss.  Bot.,  XXIX.,  '96)  Klebahn 
has  shown  that  in  the  diatoms  {Rhopalodia)  reduction  is  effected  in  the  course  of  two 
rapidly  succeeding  nuclear  divisions  occurring  before  conjugation.  After  union  of  the  conju- 
gating cells  the  nucleus  of  each  divides  twice  (maturation-divisions)  to  form  four  nuclei,  as 
in  many  Infusoria.  Each  cell  now  divides  into  a  pair  of  gametes,  each  containing  two 
nuclei;  of  the  latter  one  disappears  {cf.  the  "  corpuscules  de  rebut"),  while  the  other  forms 
the  germ- nucleus.  Each  gamete  now  conjugates  with  one  of  the  other  pair,  thus  forming 
two  zygotes  (auxospores),  the  two  germ-nuclei  in  each  fusing  to  form  a  cleavage-nucleus. 
The  number  of  "chromosomes"  in  the  two  maturation-divisions  is  four;  that  occurring  in 
the  ordinary  division  of  "  vegetative  cells "  is  certainly  larger  —  at  least  six  and  probably 
eight. 

20.  P.  200,  1.  34.  Studies  made  by  Mr.  F.  C.  Paulmier,  in  the  Columbia  laboratory,  on 
the  spermatogenesis  of  Anasa  and  some  other  Hemiptera  give  results  different  from  those 
of  Wilcox.  His  preparations  clearly  show  that  the  tetrads  arise  from  chromatin-rods  that 
undergo  a  longitudinal  split,  followed  by  a  transverse  division,  essentially  in  the  same 
manner  as  in  Gryllotalpa  or  in  the  copepods. 

21.  P.  226,  1.  20.  Several  recent  workers  seem  somewhat  disposed  to  abandon  or  to 
impose  certain  limits  to  the  law  of  genetic  continuity  in  case  of  the  centrosome;  and  they 
have  given  more  or  less  definite  accounts  of  its  free  formation.  Among  such  writers  may  be 
mentioned  Auerbach  (/.  Z.,XXX.,  '96),  Watase  {Science,  V.  no,  Feb.  5,  '97),  Child  (/.r.). 
Mead  {I.e.),  and  Lillie  {I.e.).  In  an  earlier  paper  (/.  /I/.,  XI.,  '96),  I  myself  was  inclined  to 
adopt  such  a  position,  though  with  some  reservations;  and  my  more  recent  observations 
on  sea-urchins  (see  note  to  p.  228)  may  seem  to  support  it.  I  am  nevertheless  of  Heiden- 
hain's  opinion  {Morph.  Arb.,  VII.  I,  '97.  p.  202),  that  in  this  difficult  field  of  research  the 
positive  evidence  is  entitled  to  the  foremost  place,  and  that  only  the  most  exhaustive  and 
convincing  research  will  justify  a  negative  conclusion.  The  present  controversy  regarding 
the  centrosome  recalls  the  long  debate,  following  the  promulgation  of  the  cell-theory,  on 
the  question  of  free  formation  or  genetic  continuity  in  case  of  the  nucleus.  The  outcome 
of  that  debate  may  teach  us  caution  in  this  case. 

22.  P.  228,  1.  3.  The  structure  of  the  centrosome  in  echinoderms  remains  an  open 
question.  Kostanecki's  recent  observations  {Anat.,  Hefte  VII.  2,  '96)  on  Echinus  agree 
with  those  of  Hill  on  Sphcerechimis,  the  sperm-centrosome  being  a  simple  granule  which 
divides  into  two  halves,  and  these  are  shown  in  the  plates  as  far  as  the  late  prophase  of 
cleavage.  In  Arbacia  I  have  found  essentially  similar  facts  {Science,  V.  114,  March  5,  '97), 
and  have  traced  the  centrosomes  through  the  anaphases  and  telophases,  during  which 
they  divide  and  give  rise  to  daughter  amphiasters,  as  in  7'halasscma.,  Physa,  or  Chietopterus. 


358^ 


APPENDIX 


In  ToxopnensteSy  however,  renewed  studies  indicate  that  the  facts  are  different  {Science,  Lc), 
and  they  support  in  substance  my  original  account  and  those  of  Boveri  and  Reinke.  In  this 
material,  after  the  same  treatment,  the  single  granule  is  found  only  as  late  as  the  early  pro- 
phase; at  a  later  period  the  centre  of  the  aster  is  occupied  by  a  group  (10-20)  of  intensely 
stained  granules  suspended  in  a  well-defined  reticular  "centrosphere."  The  latter  is  con- 
siderably smaller  than  in  my  former  preparations,  and  I  have  no  doubt  that  the  enormous 
spheres  shown  in  Fig.  37,  and  in  my  A/ias  of  Fertilization  have  been  exaggerated  by  the 
reagents.  I  do  not  beheve,  however,  that  the  sphere  itself  is  an  artifact,  and  I  am  inclined 
to  believe  that  it  represents  a  "  pluri-corpuscular  microcentrum,"  such  as  Heidenhain 
describes  in  the  case  of  giant-cells. 

23.  P.  240,  last  line.  There  are  probably  various  forms  of  nucleic  acid  forming  a  group 
of  nearly  related  compounds.  According  to  the  latest  work  of  Miescher  {Arch.  exp.  Path. 
u.  Pharm.,  XXXVII.,  '96),  the  formula  of  nucleic  acid  derived  from  the  sperm-nuclei  of 
the  salmon  is  C4oH54Ni40i7(P206)2-  Miescher  finds  that  the  sperm-nucleus  does  not 
consist  of  pure  nucleic  acid,  but  largely  of  protamin  nucleate,  i.e.  35.56  %  protamin 
(C16  H28N9O2)  combined  with  60.50  %  nucleic  acid. 

24.  P.  256,  1,  7.  Conklin  {Am.  Nat.,  Jan.  '97)  has  observed  similar  amoeboid  changes 
in  the  nuclei  of  intestinal  epithelium  in  isopods;  and  he  believes  that  food-granules  pass 
bodily  into  the  nucleus. 

25.  P.  278,  1.  I.  See  Jennings  {Bull.  Mus.  Comp.  Zo'ol.,  XXX.  i,  '96)  for  a  remarkable 
account  of  the  cleavage  of  the  ovum  of  Asplanchna,  in  the  course  of  which  every  known 
"  law  "  of  division  seems  to  be  contradicted. 

26.  P.  292,  1.  20.  It  is  well  known  that  leucocytosis  may  be  induced  by  various  chemical 
stimuli.  Thus  the  injection  of  nuclein  into  mammals  causes  active  leucocytosis  (Kuhman, 
Zeit.  f.  Klin.  Med.,  '95).  The  same  result  is  produced  by  pilocarpine  (Horbaczewski, 
Ber.  Wien.  Akad.,  No.  100,  '91). 

27.  P.  312,  1.  32.  This  statement  conveys  a  somewhat  misleading  impression  of  Hert- 
wig's  view.  The  "  formative  power  pervading  the  entire  mass "  is  conceived,  as  I  under- 
stand Hertwig,  not  as  a  unity,  but  is  rather  a  resultant  of  the  individual  energies  of  the 
blastomeres  and  is  due  to  their  interaction  (Wechselwirkung).  The  essence  of  Hertwig's 
conception  is  the  view  that  every  cell  contains  the  germ  of  the  whole  and  may  give  rise  to 
the  whole  or  to  any  of  its  parts,  according  to  its  relation  to  the  other  cells. 

28.  P.  239,  1.  29.  Emery  {A.  A.,  XIII.  1-2,  '97)  has  called  attention  to  the  fact  that 
the  credit  for  the  first  discovery  of  this  remarkable  regeneration  of  the  lens  belongs  to 
Vincenzo  Colucci  {Mem.  Acad.,  Bologna,  '91). 

Columbia  University,  New  York, 
March,  1897. 


INDEX   OF   AUTHORS 


Altmann,  granule-theory, ^i,  22,  28,  31,  224; 

nuclein,  240. 
Amici,  pollen-tube,  162. 
Aristotle,  epigenesis,  6. 
Arnold,  fibrillar  theory  of  protoplasm,   19; 

leucocytes,   83;    nucleus    and    cytoplasm, 

214. 
Auerbach,    5;     double    spermatozoa,    106; 

staining-reactions,  127;   fertilization,   132. 

von    Baer,    cleavage,    9;     cell-division,    46; 

egg-axis,   278;   development,  295. 
Balbiani,  spireme-nuclei,  25,  26;   mitosis  in 

Infusoria,    62;     chromatin-granules,    78; 

yolk-nucleus,    116-121;     regeneration    in 

Infusoria,   249. 
Balfour,  polar  bodies,  183;  unequal  division, 

273- 

Ballowitz,  structure  of  spermatozoa,  34,  100- 
104;   double  spermatozoa,  106. 

Van  Bambeke,  deutoplasm  and  yolk-nucleus, 
116,  117,  121;  elimination  of  chronaatin, 
116;   reduction,  173. 

Barry,  fertilization,  131. 

De  Bary,  conjugation,  163,  169;  cell-division 
and  growth,  293. 

Beale,  cell-organization,  21,  22, 

Bechamp  and  Estor,  microsome-theory,  21 ; 
microzymas,  22. 

Belajeff,  spermatozoids,  106,  107;  reduction 
in  plants,  197. 

Bellonci,  polymorphic  nuclei,  82. 

Benda,  spermatogenesis,  123,  124;  Sertoli- 
cells,  208. 

Van  Beneden,  cell-theory,  i,  4;  protoplasm, 
19;  nuclear  membrane,  28;  centrosome 
and  attraction-sphere,  36,  53,  70,  224,  230, 
232,  233;  cell-polarity,  39,  40;  cell-divi- 
sion, 46,  51,  52;  origin  of  mitotic  figure, 
53,  54;  theory  of  mitosis,  70-75;  division 
of  chromosomes,  77;  fertilization  of  As- 
caris,  134;  continuity  of  centrosomes,  143; 


germ-nuclei,  153,  154;  centrosome  in  fer- 
tilization, 157;  theory  of  sex,  183;  par- 
thenogenesis, 202;  microsomes,  213; 
nucleus  and  cytoplasm,  214;  individuality 
of  chromosomes,  217;  nuclear  microsomes, 
223;  promorphology  of  cleavage,  281 ; 
germinal  localization,  298. 

Van  Beneden  and  Julin,  first  cleavage-plane, 
280. 

Bergmann,  cleavage,  9;   cell,  13. 

Bernard,  Claude,  nucleus  and  cytoplasm, 
238,  247;  organic  synthesis,  248,  261, 
326. 

Berthold,  protoplasm,  19. 

Bickford,  regeneration  in  coelenterates,  293, 

325. 

Biondi,  Sertoli-cells,  208. 

Biondi-Ehrlich,  staining-fluid,  121. 

Bischoff,  cell,  13. 

Bizzozero,  cell-bridges,  42. 

Blanc,  fertilization  of  trout,  159. 

Blochmann,  insect-egg,  96;  budding  of 
nucleus,  117;  polar  bodies,  202 ;  bilater- 
ality  of  ovum,  283. 

Bohm,  fertilization  in  fishes,  142. 

Bolsms,  nephridial  cells,  32. 

Bonnet,  theory  of  development,  6,  328. 

Born,  chromosomes  in  Trtton-tgg,  245; 
gravitation-experiments,  285. 

Boveri,  centrosome,  named,  36,49;  a  per- 
manent organ,  56;  in  fertilization,  124, 
135,  140,  141;  continuity  of,  143;  defini- 
tion of,  224;  structure,  226,  227;  func- 
tions, 259;  archoplasm,  51,  121,  229; 
origin  of  mitotic  figure,  53,  55;  mitosis  in 
A  scar  is,  58;  varieties  of  A  scan's,  61 ;  the- 
ory of  mitosis,  71,  72;  division  of  chromo- 
somes, 77;  origin  of  germ-cells,  no,  in, 
322;  fertilization  of  Ascaris,  133,  134;  of 
Ptei'otrachea,  137;  of  Echinus,  143,  157; 
theory  of  fertilization,  140,  141 ;  of  par- 
thenogenesis,   202;     partial     fertilization. 


359 


36o 


INDEX   OF  AUTHORS 


140,  259;  reduction,  173;  maturation  in 
^j^ar/j,  179,  187;  tetrads,  221;  centriole, 
227,  235;  attraction-sphere,  233;  egg- 
fragments,  258;  position  of  polar  bodies, 
280. 

Brandt,  symbiosis,  37;  regeneration  in  Pro- 
tozoa, 248. 

Brauer,  bivalent  chromosomes,  61 ;  mitosis 
in  rhizopod,  65;  fission  of  chromatin- 
granules,  78;  deutoplasm,  117;  itxWXxz?,- 
i\on  in  Braiichipus,  142;  parthenogenesis 
in  Artemia,  156,  202-205;  spermatogene- 
sis in  Ascaris,  184,  187;  tetrads,  222; 
intra-nuclear  centrosome,  225. 

Braus,  mitosis,  74. 

Brogniard,  pollen-tube,  162. 

Brooks,  heredity,  10. 

Brown,  Robert,  cell-nucleus,  13;  pollen- 
tube,  162. 

Briicke,  cell-organization,  21,  210,  237,  249. 

von  Brunn,  spermatozoon,  102,  105. 

Buffon,  organization,  21. 

Bunting,  germ-cells,  109, 

Burger,  centrosome,  228. 

Biitschli,  5;  protoplasm,  17-19;  diffused 
nuclei,  23;  artefacts,  31;  asters,  34,  230; 
cell-membrane,  38;  mitosis,  46,  53,  75; 
centrosome  in  diatoms,  65,  224;  rejuve- 
nescence, 129;  cyclical  division,  163; 
polar  bodies,  175;  nature  of  centrosome, 
228. 

Calberla,  micropyle,  148. 

Calkins,  mitosis  in  Noctiluca,  65,  67;  yolk- 
nucleus,  1 17-121 ;  origin  of  middle-piece, 
123,  125;   reduction,  200, 

Campbell,  fertilization  in  plants,  160. 

Carnoy,  muscle-fibre,  34;  mitosis,  75;  ami- 
tosis,  81-83;   germ-nuclei,  134, 

Castle,  egg-axis,  279  ;  bilateral  cleavage,  281. 

Chittenden,  organic  synthesis,  247. 

Chmielewski,  reduction  in  Spirogyra^  199. 

Chun,  amitosis,  83;  partial  development  of 
ctenophores,  315. 

Clapp,  first  cleavage-plane,  282. 

Clarke,  mitosis  in  gregarines,  67. 

Cohn,  cell,  13. 

Conklin,  size  of  nuclei,  52;  union  of  germ- 
nuclei,  153;  centrosome  in  fertilization, 
157,  158;  unequal  division,  275;  cell-size 
and  body-size,  289. 

Corda,  pollen-tube,  162. 

Crampton,  reversal  of  cleavage,  270 ;  experi- 
ments on  snail,  315. 

Darwin,  evolution,  2,4;   inheritance,  7,  295; 


variation,  9;  pangenesis,  10,  303;  gem- 
mules,  21,  22. 

Dogiel,  amitosis,  84, 

Driesch,  dispermy,  147;  fertilization  of  egg- 
fragments,  148;  pressure-experiments,  275, 
282,309;  isolated  blastomeres,  308;  theory 
of  development,  312,  317,  328;  experi- 
ments on  ctenophores,  315;  ferment- 
theory,  327. 

Driiner,  spindle-fibres,  35;  central  spindle, 
74,  76;   aster,  234. 

Diising,  sex,  109. 

von  Ebner,  Sertoli-cells,  208. 

Eismond,  structure  of  aster,  34. 

Elssberg,  plastidules,  22. 

Endres,  experiments  on  frog's  egg,  307. 

Engelmann,   inotagmata,   22;   ciliated  cells, 

30,  31,  34;   rejuvenescence,  129. 
von    Erlanger,    asters,    34;      elimination    of 

chromatin,    117,    I2i;    fertilization,    157; 

centrosome,  228. 
Eycleshymer,  first  cleavage-plane,  282. 

Farmer,  reduction  in  plants,  196,  197. 

Fick,  fertilization  of  axolotl,  135,  142. 

Field,  formation  of  spermatozoon,  123-125: 
staining-reactions,  127. 

Fischer,  artefacts,  31,  213. 

Flemming,  protoplasm,  19,  31 ;  chromatin, 
24;  cell-bridges,  42;  cell-division,  46; 
splitting  of  chromosomes,  51;  mitotic  fig- 
ure, 53;  heterotypical  mitosis,  60;  leuco- 
cytes, 72;  theory  of  mitosis,  74;  division 
of  chromatin,  78;  amitosis,  80-84,  209; 
axial  filament,  123;  middle-piece,  125, 126; 
rotation  of  sperm-head,  137;  spermato- 
genesis, 193,  194;  astral  rays,  231;  ger- 
minal localization,  298. 

Floderus,  follicle-cells,  113. 

Fol,  I,  5,  46;  amphiaster,  49,  53;  theory  of 
mitosis,  70;  sperm-centrosome,  125;  fer- 
tilization in  echinoderms,  130,  157;  poly- 
spermy, 140;  attraction-cone,  146;  vitel- 
line membrane,  148;    asters,  230. 

Foot,  yolk-nucleus  and  polar  rings,  119,  1 21, 
150;  archoplasm,  121 ;  fertilization  in 
earthworm,  136,  143;    entrance-cone,  149. 

Foster,  cell-organization,  somacules,  22. 

Frommann,  protoplasm,  19;  nucleus  and 
cytoplasm,  214. 

Galeotti,  pathological  mitoses,  67-69. 
Galton,  inheritance,  7. 
Gardiner,  cell-bridges,  42. 
Garnault,  fertilization  in  Avion,  155. 


INDEX   OF  AUTHORS 


361 


Geddes  and  Thompson,  theory  of  sex,  90, 
Van  Gehuchten,  spireme-nuclei,  25;   nuclear 

polarity,  26;   muscle -fibre,  34. 
Giard,  polar  bodies,  177. 
Gilson,  spireihe-nuclei,  26, 
Graf,  nephridial  cells,  32. 
Griffin,  fertilization,  centrosomes  in  Thalas- 

sema^  143,  144;   structure  of  centrosome, 

235- 
Grobben,  spermatozoa,  105. 
Gruber,  diffused  nuclei,  23,  26;  regeneration 

in  Stentor^  248. 
Guignard,  mitosis  in  plants,  59,  78;   sperma- 

tozoids,  107;  fertilization   in  plants,    157, 

159,    161 ;    reduction,    195;     centrosome, 

224. 

Haacke,  gemmae,  22. 

Haberlandt,  position  of  nuclei,  252. 

Hackel,  inheritance,  5;  cell-organization, 
21,22,210;   epithelium,  40;  cell-state,  41. 

Hacker,  polar  spindles  of  Ascaris,  58;  bi- 
valent chromosomes,  61,  62;  nucleolus, 
91,93;  primordial  germ-cells,  no,  112; 
germ-nuclei,  156,  193,  194,  219;  reduc- 
tion in  copepods,  189,  191 ;  polar  bodies, 
280. 

Hallez,  promorphology  of  ovum,  283. 

Halliburton,  proteids,  239;  nuclein,  240,  241. 

Hamm,  discovery  of  spermatozoon,  7,  130. 

Hammar,  cell-bridges,  43. 

Hammarsten,  proteids,  239. 

Hansemann,  pathological  mitoses,  67,  68. 

Hanstein,  metaplasm,  15;   microsomes,  21. 

Hartsoeker,  spermatozoon,  7, 

Harvey,  inheritance,  5;    epigenesis,  6. 

Hatschek,  cell-polarity,  39,40;  fertilization, 
130. 

Heidenhain,  nucleus,  24,  25.;  basichromatin 
and  oxychromatin,  27,  244;  cell-polarity, 
39;  position  of  centrosome,  40;  leuco- 
cytes, 72,  73;  theory  of  mitosis,  74;  ami- 
tosis,  81;  staining-reactions,  127,  144; 
nuclear  microsomes,  223;  microcentrum, 
227;   asters,  234;   position  of  spindle,  277. 

Heider,  insect-egg,  96. 

Heitzmann,  theory  of  organization,  42;  nu- 
cleus and  cytoplasm,  214. 

Henking,  fertilization,  124,136;  insect-egg, 
96;   tetrads,  188;   reduction,  201. 

Henle,  granules,  21. 

Henneguy,  deutoplasm,  117. 

Hensen,  rejuvenescence,  129. 

Herhst,  development  and  environment,  324. 

Herla,  independence  of  chromosomes,  156, 
219. 


Hermann,  spermatogonia,  16;  central  spin- 
dle, 52,  74,  76;  division  of  chromatin, 
78;  spermatozoon,  123-126;  staining- 
reactions,  127;   centrosome,  224. 

Herrick,  spermatozoon,  105. 

Hertvvig,  O.,  i,  5,  7,  15,  21;  idioblasts,  22; 
cell-division,  46;  bivalent  chromosomes, 
61;  pathological  mitoses,  67;  theory  of 
mitosis,  75;  rejuvenescence,  129;  fertiliza- 
tion, 132;  middle-piece,  135;  polyspermy, 
140;  paths  of  germ-nuclei,  153;  matura- 
tion, 175,  180-182;  polar  bodies,  177; 
inheritance,  257,302;  laws  of  cell-division, 
276;  cleavage-planes,  282;  theory  of  de- 
velopment, 312,  317,  322,  328. 

Hertvvig,  O,  and  R.,  origin  of  centrosome, 
64;  egg-fragments,  145;   polyspermy,  148. 

Hertwig,  R.,  mitosis  in  Protozoa,  63,  64,  67; 
central  spindle,  74;  amphiasters  in  un- 
fertilized eggs,  159,226;  conjugation,  167; 
reduction  in  Infusoria,  199. 

Hill,  fertilization,  135,  143,  157;  centro- 
sphere,  235. 

His,  germinal  localization,  297. 

Hofer,  regeneration  in  Ai7iceba,  249. 

Hoffman,  micropyle,  148. 

Hofmeister,  cell-division  and  grovi'th,  293. 

Hooke,  R.,  cell,  13. 

Hoyer,  amitosis,  81. 

Humphrey,  centrosome,  225. 

Huxley,  protoplasm,  3;  germ,  5,  295;  fer- 
tiUzation,  129,  171 ;  evolution  and  epi- 
genesis, 328. 

Ishikawa,  Nocliluca,  mitosis,  65,  67;  conju- 
gation, 168. 

Jordan,  deutoplasm  and  yolk-nucleus,  116, 

119;   first  cleavage-plane,  282. 
Julin,  fertilization  in  Styleopsis^  142. 

Karsten,  centrosome,  225. 

Keuten,  mitosis  in  Euglena^  64. 

Klebahn,  conjugation  and  reduction  in  des- 
mids,  199. 

Klebs,  pathological  mitosis,  67,  68;  cell- 
membrane,  251. 

Klein,  nuclear  membrane,  28;  theory  of 
mitosis,  70,  230;  amitosis,  84;  nucleus 
and  cytoplasm,  214;  asters,  230. 

von  KolUker,  i,  5,  7,  9,  13;  epithelium,  40; 
cell-division,  45;  spermatozoon,  98,  122; 
inheritance,  257,  302;    development,  311. 

Korschelt,  nucleus,  25;  amitosis,  81,  83; 
movements  and  position  of  nuclei,  92, 
254-256;   insect-egg,  96;    nurse-cells.  113, 


362 


INDEX   OF  AUTHORS 


114;  ovarian  ova,  115;  fertilization,  135; 
tetrads  in  Ophryotrocha,  201 ;  physiology 
of  nucleus,  252,  254-256;  polarity  of 
egg,  287. 

Kossel,  chromatin,  241;  nuclein,  243;  or- 
ganic synthesis,  247. 

Kostanecki,  position  of  centrosome,  40. 

Kostanecki  and  Wierzejski,  fertilization  of 
Physa^  131,  136,  143,  159;  continuity  of 
centrosomes,  144;   collision  of  asters,  231. 

Krause,  polymorphic  nuclei,  82, 

Kupffer,  cytoplasm,  29. 

Lamarck,  inheritance,  10. 

Lamarle,  minimal  contact-areas,  269. 

Lankester,  germinal  localization,  297. 

Lauterborn,  mitosis  in  diatoms,  65,  67. 

Lebrun,  position  of  centrosome,  40. 

Leeuwenhoek,  spermatozoon,  7 ;  fertiliza- 
tion, 130. 

von  Lenhossek,  nerve-cell,  16,  ^y,  centro- 
some, 224. 

Leydig,  cell,  14;  protoplasm,  17;  cell-mem- 
brane, 38;  spermatozoa,  106;  elimination 
of  chromatin,  117. 

Lilienfeld,  staining-reactions  of  nucleins, 
242,  243. 

Lillie,  regeneration  in  Stentor^  249. 

Loeb,  regeneration  in  coelenterates,  293, 
325;  theory  of  development,  322;  envi- 
ronment and  development,  324. 

Lustig  and  Galeotti,  pathological  mitoses, 
68;   centrosome,  224. 

Maggi,  granules,  21. 

Malfatti,  staining-reactions  of  nucleins,  242. 

Mark,  spiral  asters,  57;    germ-nuclei,    153; 

polar    bodies,     175;     promorphology    of 

ovum,  287. 
Mathews,  pancreas-cell,  31;    fertilization  of 

echinoderms,  124,  135,  143,  157;    nucleic 

acid,  247. 
Maupas,   sex   in    Rotifers,    108;     rejuvenes- 
cence, 129;   conjugation  of  Infusoria,  165, 

168. 
LIcMurrich,  gasteropod  development,    115; 

metamerism  in  isopods,  291. 
Mead,     fertilization     of    Chcetoptej-us,     143; 

sperm-centrosome,  226. 
Merkel,  Sertoli-cells,  208. 
Mertens,  yolk-nucleus  and  attraction-sphere, 

116-121. 
Metschnikoff,  insect-egg,  284. 
Meves,  amitosis,  81-85,  209. 
Miescher,  nuclein,  240. 
Mikosch,  protoplasm,  31. 


Minot,  rejuvenescence,  129;  cyclical  divi- 
sion, 163;  theory  of  sex,  183;  Sertoli-cells, 
208;   parthenogenesis,  202. 

von  Mohl,  protoplasm,  13. 

Moore,  spermatozoon,  123-126;  reduction, 
189,  201. 

Morgan,  fertilization  of  egg-fragments,  148; 
effect  of  fertilization,  149;  numerical  rela- 
tions of  cells,  288;  isolated  blaslomeres, 
309;  experiments  on  ctenophores,  315; 
on  frog's  egg,  319. 

Nageli,  cell-organization,  21;  micellae,  22, 
301;  polioplasm,  29;  idioplasm-theory, 
300. 

Newport,  fertilization,  130;  first  cleavage- 
plane,  280. 

Niessing,  axial  filament,  123. 

Nissl,  chromophilic  granules,  33,  34. 

Nussbaum,  germ-cells,  88;  regeneration  in 
Infusoria,  248;  nucleus,  321. 

Overton,  germ-cells  of  Volvox^  98;  conjuga- 
tion of  Spirogyra,  169,  170;  reduction, 
196. 

Owen,  germ-cells,  88. 

Paladino,  cell-bridges,  42. 

Peremeschko,  leucocytes,  83. 

PfefTer,  hyaloplasm,  29;  chemotaxis  of  germ- 
cells,  145. 

Pfitzner,  cell-bridges,  42;  chromatin-gran- 
ules,  78. 

Pfliiger,  position  of  spindle,  277;  first 
cleavage-plane,  280 ;  gravitation-experi- 
ments, 285;   isotropy,  278. 

Plateau,  minimal  contact-areas,  269. 

Platner,  mitosis,  75;  formation  of  spermato- 
zoon, 123-125;  fertilization  of  Arion; 
maturation,  175,  180. 

Pouchet  and  Chabry,  development  and  en- 
vironment, 324. 

Prenant,  spermatozoon,  123. 

Preusse,  amitosis,  85,  209. 

Prevost  and  Dumas,  cleavage,  9. 

Pringsheim,  Hautschicht,  29;  fertilization, 
130. 

Purkyne,  protoplasm,  13. 

Rabl,  nuclear  polarity,  26;  cell-polarity,  39, 
40,  52;  centrosome  in  fertilization,  157; 
individuality  of  chromosomes,  215. 

Ranvier,  blood-corpuscles,  38. 

vom  Rath,  nucleus,  26;  bivalent  chromo- 
somes, 61;  amitosis,  82-84;  early  germ- 
cells,  112;    reduction,   189,   192;    tetrads, 


INDEX   OF  AUTHORS 


363 


193;   centrosome,  224;    attraction-sphere, 

234. 

Rauber,  cell-division  and  growth,  293. 

Rawitz,  spermatogonium,  15;  amitosis,  82. 

Redi,  genetic  continuity,  21. 

Reichert,  cleavage,  9,  46. 

Reinke,  pseudo-alveolar  structure,  19;  nu- 
cleus, 26,  27,  223;  oedeniatin,  28;  cyto- 
plasm, 29;  asters,  34,  226,  231 ;  central 
spindle,  74;    nucleus  and  cytoplasm,  214. 

Remak,  cleavage,  I,  9,  264;  cell-division, 
45'  46  ;   egg-axis,  279. 

Retzius,  muscle-fibre,  34  ;  cell-bridges,  42 ; 
end-piece,  104. 

Robin,  germinal  vesicle,  46. 

Rosen,  staining-reactions,  162. 

Roux,  cell-organization,  21;  meaning  of  mi- 
tosis, 51,  183,  221,  256;  position  of  spindle, 
277;  first  cleavage-plane,  277,  280;  frog- 
experiments,  mosaic  theory,  298;  theory 
of    development,    303;     post-generation, 

307- 

Riickert,  pseudo-reduction,  61,  193;  fertili- 
zation of  Cyclops^  142;  independence  of 
germ-nuclei,  156,  219;  reduction  in  cope- 
pods,  189;  early  history  of  germ-nuclei, 
193,  245;  reduction  in  selachians,  200; 
history  of  germinal  vesicle,  245. 

Riige,  amitosis,  83. 

Ryder,  staining-reactions,  127. 

Sabatier,  amitosis,  82. 

Sachs,  energid,  14 ;  laws  of  cell-division, 
265;  cell-division  and  growth,  293;  de- 
velopment, 322, 

St.  George,  La  Valette,  spermatozoon,  7,  98  ; 
spermatogenesis  (terminology),  122. 

Sala,  polyspermy,  147. 

Sargant,  reduction  in  plants,  197. 

Schafer,  protoplasm,  17. 

Scharff,  budding  of  nucleus,  1 1 7. 

Schaudinn,  mitosis  in  Amceha,  64. 

Schewiakoff,  mitosis  in  Eugiypha,  63-65. 

Schimper,  plastids,  98. 

Schleicher,  karyokinesis,  46. 

Schleiden,  cell-theory,  i ;  cell-division,  7  ; 
nature  of  cells,  13;    fertilization,  162. 

Schloter,  granules,  28,  223. 

Schmitz,  plastids,  98  ;    conjugation,  160. 

Schneider,  discovery  of  mitosis,  46. 

Schottlander,  multipolar  mitosis,  69. 

Schultze,  M.,  cells,  i,  13,  14;  protoplasm, 
19. 

Schultze,  O.,  gravitation-experiments,  285  ; 
double  embryos,  318. 

Schwann,  cell-theory,   i ;   the  egg  a  cell,  6  ; 


origin  of  cells,  7;  nature  of  cells,  13  ;  or- 
ganization, 41 ;   adaptation,  329. 

Schwarz,  protoplasm,  19;  linin,  24;  chem- 
istry of  nucleus,  28  ;  nuclei  of  growing 
cells,  246. 

Schweigger-Seidel,  spermatozoon,  7,  98,  122. 

Sedgwick,  cell-bridges,  43. 

Seeliger,  egg-fragments,  258  ;   egg-axis,  279. 

Selenka,  double  spermatozoa,  106. 

Sobotta,  fertilization  of  mouse,  136,  143. 

Solger,  pigment-cells,  73  ;  attraction-sphere, 
224. 

Spallanzani,  spermatozoa,  7. 

Spencer,  physiological  units,  21,  22  ;  devel- 
opment, 328. 

Strobe,  multipolar  mitoses,  69. 

Strasburger,  I,  5  ;  cytoplasm,  15  ;  proto- 
plasm, 19  ;  Kornerplasma,  29;  centro- 
sphere,  49,  232  ;  origin  of  amphiaster, 
53  ;  multipolar  mitoses,  69 ;  theory  of 
mitosis,  74,  76,  77  ;  amitosis,  83  ;  sper- 
matozoids,  107,  108,  126  ;  kinoplasm,  108, 
126;  staining-reactions  of  germ-nuclei, 
128;  fertilization  in  plants,  135,  160, 
162  ;  reduction,  188,  195  ;  theory  of 
maturation,  196;  organization,  210;  in- 
heritance, 257,  302  ;  action  of  nucleus, 
322. 

zur  Strassen,  primordial  germ-cells  in  Asca- 
ris.  III, 

Van  der  Stricht,  amitosis,  82 ;  attraction- 
sphere,   224  ;   fertilization  in  Amphioxus, 

159- 
Stuhlmann,  yolk-nucleus,  1 19. 

Tangl,  cell-bridges,  42. 

Thiersch  and  Boll,  theory  of  growth,  292. 

Treat,  sex,  109. 

Ussow,  micropyle,  97;   deutoplasm,  117. 

Vejdovsky,  centrosome,  55  ;  fertilization  in 
Rhynchelmis^  142  ;  metamerism  in  an- 
nelids,  291. 

Verworn,  cell-physiology,  4 ;  cell-organiza- 
tion, 21  ;  biogens,  22  ;  regeneration  in 
Protozoa^  249  ;  nucleus  and  cytoplasm, 
252  ;   inheritance,  327. 

Virchow,  I  ;  cell-division,  8,  9,  21,  45-47  ; 
protoplasm,  19  ;   cell-state,  41. 

De  Vries,  organization,  21  ;  pangens,  22  ; 
tonoplasts,  37  ;  plastids,  170  ;  chromatin, 
183  ;  panmeristic  division,  236  ;  pangene- 
sis, 303;   development,  312. 

Waldeyer,  nucleus,  27  ;  cytoplasm,  29;  cell- 


364 


INDEX   OF  AUTHORS 


membrane,  38  ;  chromosomes,  47  ;  amito- 
sis,  83. 

Walter,  frog-experiments,  307. 

Wasielewsky,  centrosome,  225. 

Watase,  theory  of  mitosis,  75  ;  staining- 
reactions  of  germ-nuclei,  127;  nucleus  and 
cytoplasm,  211  ;  asters,  226;  theory  of 
centrosome,  228  ;  astral  rays,  231  ;  cleav- 
age of  squid,  273,  283  ;  promorphology 
of  ovum,  283,  287. 

Weismann,  inheritance,  10,  ii,  302;  cell- 
organization,  21;  biophores,  22;  ids,  27; 
somatic  and  germ  cells,  88;  amphimixis, 
130;  maturation,  183-185;  constitution 
of  the  germ-plasm,  183,  305;  partheno- 
genesis, 202 ;  theory  of  development,  303- 
305*  328. 

Went,  vacuoles,  37. 

Wheeler,  amitosis,  81;  insect-egg,  97; 
fertilization  in  Myzostoma^  I57~i59; 
plastids,  170  ;  bilaterality  of  ovum, 
283. 

Whitman,  on  Harvey,  6  ;  cell-organization, 
21  ;  idiosome,  22  ;  polar  rings,  150  ;  cell- 
division  and  growth,  293  ;  theory  of  de- 
velopment, 297,  299. 

Wiesner,  cell-organization,  21,  137;  pla- 
some,  22  ;  panmeristic  division,  236. 


Wilcox,  sperm-centrosome,  1 23,  1 24 ;  re- 
duction,  189,   200. 

Will,  chromatin-elimination,  117. 

Wilson,  fertiUzation  in  sea-urchin,  135,  136, 
143;  paths  of  germ-nuclei,  152;  origin 
of  linin,  223  ;  astral  rays,  231  ;  centro- 
sphere  and  centrosome,  232-235  ;  di- 
spermy,  260  ;  pressure-experiments,  309  ; 
first  cleavage-plane,  277  ;  experiments  on 
Aviphioxus^  308,  319;  theory  of  develop- 
ment, 317. 

von  Wittich,  yolk-nucleus,  118. 

Wolff,  C.  F.,  epigenesis,  6, 

Wolff,  G.,  regeneration  of  lens,  329. 

Wolters,  mitosis  in  gregarines,  67 ;  polar 
body  in  gregarines,   199. 

Yung,  sex,  109. 

Zacharias,  E.,  nucleoli,  24,  25  ;  of  meristem, 
27;  staining-reactions,  127;  nuclein  in 
growing  cells,  246. 

Zacharias,  O.,  amoeboid  spermatozoa,  105. 

Ziegler,  artificial  mitotic  figure,  75  ;  amito- 
sis, 83,  84. 

Zimmerman,  pigment-cells,  73. 

Zoja,  independence  of  chromosomes,  156, 
219  ;   isolated  blastomeres,  309. 


INDEX   OF   SUBJECTS 


Achromatic  figure  (see  Amphiaster),  50; 
varieties  of,  57;   nature,  229. 

Adinosphccrium,  mitosis,  63,  66;  regenera- 
tion, 248. 

Adaptation,  329, 

y^quorea^  metanucleus,  93. 

Albumin,  239,  241. 

Allolobophora^  fertilization,  136;  teloblasts, 
274. 

Amphiaster,  49;  asymmetry  of,  51,  275; 
origin,  49,  75,  231;  in  amitosis,  81 ;  in 
fertilization,  134,  140,  142,  156;  nature, 
260;   position,  275-277. 

Amitosis,  46,  80;  biological  significance,  82; 
in  sex-cells,  209. 

Amoeba^  4;  mitosis,  64;  experiments  on, 
249. 

Amphibia,  spermatozoa,  100. 

Amphimixis,  130,  171. 

Amphioxtis^  fertilization,  153,  159;  polar 
body,  176;  cleavage,  270,  271;  dwarf 
larvae,  289,  307;   double  embryos,  308. 

Aniphipyrenin,  29. 

Amyloplasts,  37;  in  plant-ovum,  98,  160. 

Anaphases,  47,  51 ;   in  sea-urchin  egg,  77. 

Anilocra^   gland-cells,  nuclei,  26;   amitosis. 

84.  _ 

Anodonta,  ciliated  cells,  30. 

Antipodal  cone,  71. 

Archoplasm,  51;  in  developing  sperma- 
tozoa, loi,  123,  126,  in  spermatozoids, 
108,  126;  and  yolk-nucleus,  121 ;  nature 
of,  229-231. 

Argonaiita,  micropyle,  97. 

Arion^  germ-nuclei,  155. 

Artefacts,  in  protoplasm,  31,  213. 

Artemia,  chromosomes,  49,  61,  205;  par- 
thenogenetic  maturation,  202-205. 

Ascaris,  chromosomes,  49;  mitosis,  52,  58, 
71,  78;  primordial  germ-cells,  no,  332; 
fertilization,  132-134,  141  ;  polyspermy, 
147;   polar  bodies,  179;   spermatogenesis, 


180-182,  184;  individuality  of  chromo- 
somes, 215-218;  intranuclear  centrosome, 
225;  attraction-sphere,  233;  supernum- 
erary centrosome,  259. 

Aster,  34;  asymmetry,  51  ;  spiral,  57; 
structure  and  functions,  71;  in  amitosis, 
81;  in  fertilization,  138,  157;  nature  of, 
229-231 ;  finer  structure,  231,  233,  244; 
relative  size,  275. 

Asterias^  spermatozoa,  127;  sperm-aster, 
140 ;   fertilization,  143,  146. 

Astrocentre,  232. 

Astrosphere,  232. 

Attraction-cone,  146. 

Attraction-sphere,  36,  53,  54;  in  amitosis, 
81;  of  the  ovum,  119;  of  the  spermatid, 
125;  in  resting  cells,  224;  nature  of, 
232-235. 

Axial  filament,  99;   origin  of,  123. 

Axis,  of  the  cell,  38;  of  the  nucleus,  26, 
215;   of  the  ovum,  40,  278-280,  298,  319. 

Axolotl,  fertilization,  131. 

Bacteria,  nuclei,  23. 

Basichromatin,  27,  223;  staining-reactions, 
223,  245. 

Bioblast,  22. 

Biogen,  22. 

Biophore,  22,  183,  305. 

Birds,  blood-cells,  46;  spermatozoa,  102; 
young  ova,  119. 

Blastomeres,  displacement  of,  270;  in- 
dividual history,  273;  prospective  value, 
280,  313;  rhythm  of  division,  290;  de- 
velopment of  single,  298,  307-309,  315, 
319;   in  normal  development,  312. 

Bleufiius,  pigment-cells,  73. 

Branchipus^  yolk,  117;  sperm-aster,  142; 
reduction,  188. 


Calanus,  tetrads,  190. 
Cancer-cells,  mitosis,  68. 


365 


;66 


INDEX   OF  SUBJECTS 


Canthocamptus^    reduction,     190 ;     ovarian 

eggs,  193'  194- 

Cell,  in  general,  3;  origin,  7,  8;  name,  13; 
general  sketch,  14;  polarity  of,  t^Z;  as  a 
structural  unit,  41 ;  structural  basis,  16-22, 
212;  physiology  and  chemistry,  238;  size 
and  numerical  relations,  289-292;  in  in- 
heritance, 7,  295,  328;  differentiation  of, 
311-315;   independence  of,  323. 

Cell-bridges,  42. 

C^ell-division  (see  Mitosis,  Amitosis),  general 
signilicance,  9,  45;  general  account,  45; 
types,  46;  Remak's  scheme,  46;  indirect, 
47;  direct,  80;  cyclical  character,  129, 
164;  equal  and  reducing  or  qualitative, 
185,  304,  305;  relation  to  development, 
264;  Sachs's  laws,  265;  rhythm,  268,  289; 
unequal,  270-276;  of  teloblasts,  271; 
energy  of,  289;  relation  to  metamerism, 
291  ;  causes,  292;  relation  to  growth, 
293;  and  differentiation,  312,  323. 

Cell-membrane,  38. 

Cell-organs,  37;  nature  of,  21 1 ;  temporary 
and  permanent,  211,  236. 

Cell-organization,  21 ;  general  discussion, 
210-237. 

Cell- plate,  52. 

Cell- state,  41. 

Cell-theory,  general  sketch,  i-io. 

Central  spindle,  49,  52;   origin  and  function, 

74- 

Centrosome,  17;  general  sketch,  36;  posi- 
tion, 39;  in  mitosis,  49;  a  permanent 
organ,  54,  224;  dynamic  centre,  56; 
historical  origin,  67;  functions,  70,  259; 
in  amitosis,  81 ;  of  the  ovum,  91  ;  of  the 
spermatozoon,  99,  lor,  123;  in  fertiliza- 
tion, 135,  141,  144,  156-159,  171;  degen- 
eration of,  135,  142,  171,  224;  continuity, 
143,  227;  in  parthenogenesis,  156,  203; 
nature,  224-229;  intra-nuclear,  64,  225; 
effect  on  cytoplasm,  36,  212;  supernum- 
erary, 260. 

Centrosphere,  36,  49,  77;   nature  of,  232. 

Ceratozainia,  reduction,  196. 

Cerianthtis,  regeneration  in,  293. 

Chatopterus^  fertilization,  143;  sperm-centro- 
some,  226. 

Cham,  spermatozoids,  106. 

Chironomus,  spireme-nuclei,  26. 

Chorion,  96. 

Chromatic  figure,  50;  origin,  53;  varieties, 
59;   in  fertilization,  134. 

Chromatin,  24;  in  meristem,  27;  in  mitosis, 
47;  in  cancer-cells,  68;  of  the  egg-nucleus, 
92;    elimination  of,  in  cleavage,    m,   in 


oogenesis,  117,  1 21;  staining-reactions, 
127,  243,  244;  morphological  organization, 
78,  80,  183-185,  215-222,  304,  305; 
chemical  nature,  28,  241-244;  relations 
to  linin,  223;  physiological  changes,  244- 
247;  as  the  idioplasm,  257,  301,  302;  in 
development,  321,  322,  326. 

Chromatin-granules,  27;  in  mitosis,  78;  in 
reduction,  206;  general  signiHcance,  221, 
222,  305 ;   relations  to  linin,  223. 

Chromoplast,  37. 

Chromatophore,  37,  211  ;  in  the  ovum,  98; 
in  fertilization,  169. 

Chromomere  (see  Chromatin-granule),  27, 
221. 

Chromosomes,  27;  in  mitosis,  47-52;  num- 
ber of,  48,  49,  154,  219;  variation  of,  59; 
bivalent  and  plurivalent,  61,  190,  205- 
207;  division,  77;  of  the  primordial  germ- 
cell,  III;  in  fertilization,  134,  135,  154; 
independence  in  fertilization,  156,  219; 
reduction,  173;  in  early  germ-nuclei,  193; 
conjugation  of,  199;  in  parthenogenesis, 
203,204;  individuality  of,  215-221 ;  com- 
position of,  221,  304,  305;  chemistry, 
243;  history  in  germinal  vesicle,  245;  in 
dwarf  larvae,  258. 

Ciliated  cells,  30,  34. 

Ciona,  egg-axis,  280. 

Clavelina^  cleavage,  270,  281, 

Cleavage,  in  general,  9;  geometrical  rela- 
tions, 265-278;  Sachs's  laws,  265;  modi- 
fications of,  268;  spiral,  270 ;  reversal  of, 
270;  meroblastic,  271 ;  under  pressure, 
275,  309;  Hertwig's  laws,  276;  promor- 
phology  of,  278;  bilateral,  280;  rhythm, 
290;  mosaic  theory,  299;  half  cleavage, 
308;  and  development,  309-320,  323; 
partial,  315. 

Cleavage-nucleus,  153. 

Cleavage-planes,  267;  determination  of,  277; 
axial  relations,  280-285,  '^'^1- 

Clepsine,  nephridial  cell,  t^2',  polar  rings, 
150;   cleavage,  270. 

Closterium^  conjugation  and  reduction,  198. 

Cockroach,  amitosis,  81;  orientation  of  egg, 
283. 

Ccelenterates,  germ-cells,  109;   regeneration, 

325. 

Conjugation,  in  unicellular  animals,  163-168; 
unicellular  plants,  169;  physiological  mean- 
ing, 129,  165. 

Contractility,  theory  of  mitosis,  70-74;  in- 
adequacy, 77. 

Copepods,  reduction,  190. 

Corixa,  ovum,  284. 


INDEX   OF  SUBJECTS 


l(>7 


Crepidula^  fertilization,  157;  position  of 
spindles,  277;    dwarfs  and  giants,  289. 

Cross-furrow,  270. 

Crustacea,  spermatozoa,  105,  106. 

Ctenophores,. experiments  on  eggs,  314. 

Cyclas,  ciliated  cells,  30. 

Cyclops^  ova,  93 ;  primordial  germ-cells,  1 1 2 ; 
fertilization,  142,  156,  218;  reduction,  189- 
191 ;  attraction-sphere,  233;  axial  rela- 
tions, 286. 

Cytolymph,  17. 

Cytoplasm,  15,  29,  213,  236;  of  the  ovum, 
97,  115,  170;  of  the  spermatozoon,  107; 
morphological  relations  to  nucleus,  214; 
to  archoplasm,  230-235;  chemical  rela- 
tions to  nucleus,  238,  240,  241;  physio- 
logical relations  to  nucleus,  248;  in 
inheritance,  297,  298,  327;  in  develop- 
ment, 315-320;   origin,  327. 

DendrobcBfia,  metamerism,  291. 

Determinants,  183,  305. 

Deutoplasm,  90,  91,  94;  deposit,  115;  ar- 
rangement, 117,  279;  effect  on  cleavage, 
273;   re-arrangement  by  gravity,  285,  319. 

Development,  I,  6;  and  cell-division,  264; 
mosaic  theory,  298;  theory  of  Nageli,  301; 
Roux-Weismann  theory,  303;  of  single 
blastomeres,  298,  307,  315;  of  egg-frag- 
ments, 217,  258,  285,  315;  Hertwig's 
theory,  312,  317;  Driesch's  theory,  313, 
317;  partial,  315;  half  and  whole,  319; 
nature  of,  320;  external  conditions,  323; 
and  metabolism,  326;  unknown  factor, 
327;     rhythm,   328;     adaptive    character, 

329- 
Diatoms,  mitosis,  67;    centrosome,  224. 
Diemydyhis^  yolk,  116;   yolk-nuclei,  119. 
Differentiation,  264,  296;  theory  of  De  Vries, 

303;      of    Weismann,    305;     nature    and 

causes,  311-320;  of  the  nuclear  substance, 

321;   and  cell-division,  323. 
Dispermy,  147,  260. 
Double  embryos,  308,  319. 
Dwarfs,    formation    of,    258,   289,   307-309, 

315;   size  of  cells,  289. 
Dyads  (Zweiergrappen),  179,  181,  184,  189; 

in  parthenogenesis,  203-205. 
Dyaster,  51. 

Dycyemids,  centrosome,  36. 
Dytisciis,  ovarian  eggs,  1 15,  256. 

Earthworm,  ova,  115;  spermatozoon,  125; 
yolk-nucleus,  121;  fertilization,  135;  polar 
rings,  150;  spermatogenesis,  200;  telo- 
blasts,  274. 


Echinoderms,  spermatozoa,  123;  fertiliza- 
tion, 143,  157;  polyspermy,  147;  dwarf 
larvae,  258,  289;  half  cleavage,  306;  eggs 
under  pressure,  309;   modified  larvae,  324. 

Echinus,  fertilization,  143, 157;  centrosome, 
235;  dwarf  larvae,  258;  number  of  cells, 
291. 

Egg-axis,  278;  promorphological  signifi- 
cance, 279,  298;  determination,  285,  287, 
322;   alteration  of,  319. 

Egg-centrosome,  91,  119;  degeneration  of, 
91,  138,  141,  142,  171;  asserted  persist- 
ence, 157-159. 

Egg-fragments,  fertilization,  97,  145,  148; 
development,  217,  258,  285,  289,  315. 

Elasmobranchs,  spermatozoon,  100,  124; 
germinal  vesicle,  193,  245;   reduction,  200. 

Embryo-sac,  160. 

Enchylema,  17. 

Endoplasm,  29. 

End-piece,  100,  104. 

End-plate,  64. 

Envelopes,  of  the  egg,  96. 

Epigenesis,  6,  305,  327,  328. 

Equatorial  plate,  49,  58,  66;   formation,  74. 

Euchceta,  tetrads,  190. 

Euglena,  mitosis,  64. 

Euglypha,  mitosis,  63,  277. 

Evolution  (preformation),  6,  298,  305,  327, 
328. 

Evolution,  theory  of,  3. 

Exoplasm,  29. 

Fertilization,  general  aspect,  7;  physiologi- 
cal meaning,  129;  general  sketch,  130; 
Ascaris,  132;  mouse,  136;  sea-urchin, 
138;  Nereis,  14 1 ;  Cyclops,  142;  Thalas- 
sema,  Chcetoptertis,  143;  pathological,  148; 
partial,  140,259;  oi  Afyzostoma,  159;  in 
plants,  160;  egg-fragments,  97,  145,  258; 
Boveri's  theory,  141 ;  general  aspect,  171 ; 
Minot's  theory,  183;    Maupas  on,  165. 

Fishes,  pigment-cells,  73;  periblast-nuclei, 
83;  spermatozoa,  100;  young  ova,  116; 
dwarfs,  309. 

Flagellates,  diffused  nuclei,  23. 

Follicle,  of  the  egg,  113. 

Eorjicula,  nurse-cells,  115,  255. 

Fragmentation,  46. 

Fritillaria^  spireme,  78. 

Frog,  ganglion-cell,  16,  33;  tetrads,  192; 
egg-axis,  278;  first  cleavage-plane,  280, 
282,  298 ;  Roux's  puncture  experiment, 
299;  post-generation,  307;  pressure-ex- 
periments, 309;  effect  of  gravity  on  the 
I        egg,    285,    319;     development    of    single 


368 


INDEX   OF  SUBJECTS 


blastomeres,  299,   319;    double  embryos, 
319- 

Ganglion-cell,  T,y,   centrosome  in,  16,  224. 

Gemmae,  22. 

Gemmules,  22,  303. 

Genoblasts,  183. 

Geophiliis,    deutoplasm,    116;    yolk-nucleus, 

119. 
Germ,  5,  295. 
Germ-cells,  general,  7-1 1 ;   detailed  account, 

88;   of  plants,  97,  106;   origin  and  growth, 

108;      growth    and    differentiation,    1 13; 

union,    130,    145;    results  of  union,   149; 

maturation,   173;    early  history  of  nuclei, 

193;   in  inheritance,  295. 
Germinal  localization,    theory   of,  296-300, 

317- 

Germinal  spot,  91. 

Germinal  vesicle,  90;  structure,  92;  in  ma- 
turation, 179;  early  history,  193,  245; 
movements,  150,  255;   position,  287. 

Germ-nuclei,  of  the  ovum,  92;  of  the  sper- 
matozoon, 99,  103;  of  plants,  106,  162; 
staining-reactions,  127,  162;  in  fertiliza- 
tion, 132,  153, 160,  257;  equivalence,  132, 
155,  171 ;  paths,  151 ;  movements,  153; 
union,  53;  independence,  156,  219;  in 
Infusoria,  165;   early  history,  165. 

Giant-cells,  25;   microcentrum,  227,  228. 

Globulin,  239,  241. 

Granules  (see  Microsomes),  of  Altmann,  21; 
nuclear,  28;  chromophilic,  34;  in  general, 
223. 

Gravity,  effect  on  the  egg,  96,  286,  319. 

Gregarines,  mitosis,  67;   polar  body,  199. 

Ground-substance,  of  protoplasm,  19;  of 
nucleus,  25. 

Growth,  and  cell-division,  41,  278,  293,  312; 
laws  of,  292, 

Gryllotalpa,  reduction,  188. 

Heterocope,  tetrads,  190. 

Heterokinesis,  305. 

Histon,  243. 

Homcjeokinesis,  305. 

Jlydrophilns,  orientation  of  egg,  283. 

Id,  27;    in    reduction,   185;   in  inheritance, 

305- 
Idant,  305. 
Idioblast,  22,  328. 
Idioplasm,    theory   of,    300 ;    as   chromatin, 

302;   action  of,  301,  327,  329. 
Idiosome,  22. 
Ilyanassa,  partial  development,  317. 


Infusoria,  nuclei,  23,  165;  mitosis,  62;  con- 
jugation, 164;    reduction,  199. 

Inheritance,  of  acquired  characters,  10,  329; 
Weismann's  theory,  1 1 ;  through  the  nu- 
cleus, 5,  135,  248,  257,  262,  302,  327;  and 
metabolism,  326. 

Inotagmata,  22. 

Insect-eggs,  96,  283,  284. 

Interzonal  fibres,  51,  74. 

Isopods,  metamerism,  291. 

Isotropy,  of  the  egg,  285,  287,  312,  315. 

Karyokinesis  (see  Mitosis),  46,  47, 
Karyokinetic  figure  (see  Mitotic  figure),  50. 
Karyoplasm,  15. 
Karyosome,  24. 

Kinoplasm  (archoplasm),  in  spermatozoids, 
108,  126. 

Lanthanin,  27. 

Leucocytes,  structure,  72;  division,  83;  cen- 
trosome, 224,  227;   attraction-sphere,  234. 

Leucoplasts,  of  plant-ovum,  98, 

Lilium,  mitosis,  59;  spireme,  78;  fertiliza- 
tion, 160;   reduction,  195-197. 

Limax,  spiral  asters,  57;   germ-nuclei,  153. 

Linin,  24,  28;  relations  to  cytoreticulum  and 
chromatin,  214,  223. 

Locusta,  orientation  of  egg^  283. 

Loligo,  cleavage,  273,  282,  283. 

Macrogamete,  167. 

Macromeres,  273,  311,  313. 

Mammals,  spermatozoa,  104;  young  ova, 
119. 

Mantle-fibres,  52,  74. 

Maturation  (see  Reduction),  131 ;  theoreti- 
cal significance,  182;  of  parthenogenetic 
eggs,  202;   nucleus  in,  259. 

Medusas,  dwarf  embryos,  309. 

Meristem,  nuclei  of,  246. 

Metamerism,  291. 

Metaphase,  47. 

Metaplasm,  15. 

Micellae,  22,  301,  327. 

Microcentrum,  227. 

Microgamete,  167. 

Micromeres,  273,  311,  313. 

Micropyle,  90,  97. 

Microsomes,  21;  of  the  egg-cytoplasm,  94; 
nature  of,  212,  228,  237;  of  the  astral 
systems,  213,  214;  of  the  nucleus,  214, 
223;  relation  to  centrosome,  229;  stain- 
ing-reactions, 244. 

Microzynia,  22. 

Mid- body,  52,  56. 


INDEX   OF  SUBJECTS 


369 


Middle-piece,  99,  100,  103;  origin,  123, 
125;   in  fertilization,  135,  137,  143,  157. 

Mitosis,  46;  general  outline,  47;  modifica- 
tions of,  57;  heterotypical,  60;  in  uni- 
cellular ^orms,  62;  pathological,  67; 
multipolar,  69;  mechanism  of,  70—75; 
physiological  significance,  86,  171,  183, 
256;  in  fertilization,  134;  Roux-Weismann 
conception  of,  104,  305. 

Mitosome,  123. 

Mitotic  figure  (see  Mitosis,  Spindle),  50; 
origin,  53;   varieties,  57. 

Mouse,  formation  of  spermatozoon,  126; 
fertilization,    136, 

Musca,  ovum,  96. 

Myriapods,  spermatozoa,  106;  yolk-nucleus, 
117. 

Myzostoma^  fertilization,  159. 

Nebenkern,  pancreas-cells,  31;  archoplas- 
mic,  53;   of  spermatid,  123,  125. 

A^ecturiis,  pancreas-cells,  31. 

Nematodes,  germ-nuclei,  134. 

Nereis^  asters,  34;  perivitelline  layer,  94; 
ovum,  95;  deutoplasm,  96;  fertilization, 
141 ;  sperm-centrosome,  226;  attraction- 
sphere  and  centrosome,  233;  cleavage, 
271,  276;  pressure-experiments  on,  309, 
310. 

Nerve-cell,  2)2>- 

Net-knot,  24. 

Noctiluca^  mitosis,  65;  conjugation,  168. 

Nuclear  stains,  28,  242. 

Nucleic  acid,  29,  240;  staining-reactions, 
243;   physiological  significance,  247. 

Nuclein,  17,  29,  239,  240;  staining-reac- 
tions, 242-246;  physiological  significance, 
247. 

Nucleo-albumin,  239  ;  relations  to  nuclein, 
241,  243. 

Nucleo-proteid,  242,  243. 

Nucleolus,  14,  24;  in  mitosis,  49;  of  the 
ovum,  91-93;  physiological  meaning,  93; 
relation  to  centrosome,  64,  225. 

Nucleoplasm,  15. 

Nucleus,  general  structure  and  functions, 
22;  finer  structure,  27;  polarity,  26,  215; 
chemistry,  28;  in  mitosis,  47,  256;  of  the 
ovum,  92;  of  the  spermatozoon,  98,  103; 
relation  to  cytoplasm,  214,  238;  morpho- 
logical composition,  215;  in  organic  syn- 
thesis, 247,  262;  physiology,  248;  position 
and  movements,  252-256;  in  fertilization, 
257;  in  maturation,  259;  in  later  devel- 
opment, 321,  327;  in  metabolism  and 
inheritance,   326;   in  inheritance  and   de- 


velopment, 302-311,  317;    control  of  the 
cell,  322. 
Nurse-cells,  113,  114,  255. 

CEdigonium,   fertilization,    130;    membrane, 

252, 
Oocyte,  175,  176. 
Oogenesis,  173,  175. 
Oogonium,  175,  176. 
Oosphere,  97.  « 

Ophryotrocha,  amitosis,  81 ;  nurse-cells,  113; 

tetrads,  192,  201. 
Organism,  nature  of,  41. 
Organization,  41,  210;  of  the  nucleus,  222; 

of  the  cytoplasm,  223;  of  the  egg,  299, 

327. 

Origin  of  species,  4. 

Osimmda^  reduction,  196. 

Ovary,  89;   of  Canthocamptus,  194. 

Ovum,  in  general,  5,  6;  detailed  account, 
90;  nucleus,  92;  cytoplasm,  94;  envel- 
opes, 96;  of  plants,  97;  origin  and  growth, 
113;  fertilization,  129;  effects  of  spermato- 
zoon upon,  149;  maturation,  175;  parthe- 
nogenetic,  202;  promorphology,  278,  285; 
bilaterality,    282;     in    development,    311, 

323- 
Oxychromatin,   27,   223;    staining-reactions, 

244. 
Oyster,  germ-nuclei,  staining-reactions,  127. 

Pallavicinia,  chromosomes,  49  ;  reduction, 
196. 

Paludina^  dimorphic  spermatozoa,  104,  106. 

Pangens,  22,  303,  305,  312,  322,  328. 

Pangenesis,  10,  303,  305,  312. 

Parachromatin,  29. 

Paralinin,  29. 

Paramcccium^  mitosis,  62;  conjugation,  165; 
reduction,  199. 

Paranucleus,  53;   of  the  spermatid,  123. 

Parthenogenesis,  centrosome  in,  156;  preven- 
tion of,  91,  176;  theories  of,  202;  polar 
bodies  in,  202-205. 

Petromyzon,  fertilization,  142,  147. 

Phalhisia,  fertilization,  143,  153;  centro- 
sphere,  235. 

Physa,  fertilization,  131,  144;  reversed  cleav- 
age, 270. 

Physiological  units,  22. 

Pieris,  spinning-gland,  25,  254. 

Pigment-cells,  72. 

Pilidaria,  fertilization,  160. 

Pinits,  reduction,  196. 

Plant-cells,  plastids,  37;  membranes,  38; 
mitosis,  57,  59;   cleavage-planes,  266. 


J/' 


INDEX   OF  SUBJECTS 


Plasma-stains,  28,  242. 

Plasmosome,  24. 

Plasome,  22. 

Plastids,  37;  of  the  ovum,  98,  160;  of  the 
spermatozoid,  107;  conjugation  of,  169; 
in  fertilization,  170,  171 ;  independence, 
211. 

Plastidule,  22. 

Polar  bodies,  131 ;  nature  and  mode  of 
formation,  175-180 ;  division,  179;  in 
parthenogenesis,  202-205. 

Polarity,  of  the  nucleus,  26,  215;  of  the  cell, 
38;  of  the  ovum,  278,  279,  298;  determi- 
nation of,  285,  287,  322. 

Polar  rings,  121,  150. 

Pole-plates,  64. 

Pollen-grains,  formation,  59;   division,   195. 

Pollen-tube,  106,  161,  162. 

Polyclades,  cleavage,  313. 

Polygordius,  cleavage,  269. 

Polyspermy,  140,  147;   prevention  of,  148. 

Polystomella,  regeneration,  249,  250. 

Porcellio,  amitosis,  82. 

PredeUneation,  297. 

Preformation  (see  Evolution). 

Principal  cone,  71. 

Promorphology  (see  Cleavage,  Ovum). 

Pronuclei,  151. 

Prophase,  47. 

Proteids,  239. 

Prothallium,  160;    chromosomes  in,  196. 

Protoplasm,  3,  15;  structure,  17,  212;  chem- 
istry, 238. 

Protoplast  (see  Plastid). 

Pseudo-alveolar  structure,  19,  94. 

Pseudo-reduction,  61,  193,  194,  197,  205. 

Pterotrachea,  germ-nuclei,  135,  137,  153. 

Ptychoptera,  spireme-nuclei,  25. 

Pyg(E}'a,  formation  of  spermatozoon,  123. 

Pyrenin,  25,  29. 

Pyrenoid,  98. 

Pyrrochoris,  tetrads,  188. 

Quadrille  of  centres,  157. 

Reduction,  general,  173;  general  outline, 
174;  parallel  between  the  two  sexes,  180, 
182;  theoretical  significance,  182-185; 
detailed  account,  186-193;  in  plants, 
195-197;  Strasburger's  theory  of,  196; 
in  unicellular  forms,  198;  by  conjugation, 
199;   modes  contrasted,  206. 

Regeneration,  293,  294;  Weismann's  theory, 
305;  in  frog-embryo,  307;  nature  of,  323; 
in  coelente rates,  225;   of  lens,  329. 

l<.ejuvenescence,  129,  165. 


Renilla,  ovum,  96,  145. 
Rhabdonema,  amitosis,  81. 
Rhynchelmis,    fertilization,    142;     cleavage, 
272. 

Sagitta,  number  of  chromosomes,  49;  pri- 
mordial germ-cells,  no;  germ-nuclei,  135; 
sperm-aster,  140. 

Salamander,  epidermis,  2,  16,  20;  sperma- 
togonia, 15,  16,  234;  nuclei,  24;  mitosis 
in,  54-56,  60;  pathological  mitosis,  69; 
leucocytes,  72;  spermatocyte,  78;  amito- 
sis, 82;  spermatozoa,  125;  tetrads,  191, 
192. 

Sargus,  pigment-cells,  73. 

Segmentation  (see  Cleavage). 

Selaginella.    spermatozoids,  145. 

Senescence,  130,  165. 

Sertoli-cells,  183,  208. 

Sex,  7;  determination  of,  109;  nature  of, 
130;  Minot's  theory  of,  183,  208. 

Siphonophores,  amitosis,  Z^. 

Soma,  II. 

Somacule,  22. 

Somatic  cells,  88;  number  of  chromosomes, 
176. 

Spermary,  89. 

Spermatid,  122,  180;  development  into 
spermatozoon,  1 22- 1 26. 

Spermatocyte,  122,  180;  of  Ascaris,  180, 
225. 

Spermatogenesis  (see  Reduction),  173;  gen- 
eral outline,  parallel  with  oogenesis,  180- 
182. 

Spermatogonium,  122,  180;  early  history, 
194;   of  salamander,  234. 

Spermatozeugma,  106. 

Spermatozoid,  structure  and  origin,  106, 
126;   in  fertilization,  145,  160. 

Spermatozoon,  discovery,  7;  structure,  98; 
essential  parts,  loi ;  giant,  105 ;  double, 
106  ;  unusual  forms,  106;  of  plants,  107  ; 
formation,  123  ;  in  fertilization,  131,  136; 
entrance  into  ovum,  145;  physiological 
significance,  90,  141,  171. 

Sperm-centrosome,  99,  loi  ;  position,  123; 
in  fertilization,  135,  138,  141,  143,  144, 
156-159,  171. 

Sperm-nucleus,  99,  loi,  103;  origin,  122; 
in  fertilization,  132,  153;  rotation,  137; 
path  in  the  egg,  151 ;  in  inheritance,  252, 

257- 
Sphar echinus,  fertilization,  143  ;  number  of 

cells,  291  ;  hybrids,  258. 
Spindle   (see  Amphiaster,  Central  spindle), 

49^  57;  origin,  48,  53,  74,  214;  in  Pro- 


INDEX   OF  SUBJECTS 


371 


tozoa,  64-67  ;  conjugation  of,  168  ;  nature 
of,  230,  231;   position,  276-278. 

Spireme,  27,  47,  77,  193. 

Spirochona,  mitosis,  62,  63. 

Spij-ogyra,  donjugation,  169;  reduction, 
199. 

Spongioplasm,  17. 

Spontaneous  generation,  5. 

Stem-cells,  iii,  112. 

Stentor,  regeneration,  249,  250. 

Stylonychia,  senescence,  165. 

Symbiosis,  211. 

Synapta^  cleavage,  267,  268. 

Syncytium,  42. 

Telophase,  47,  52. 

Teloblasts,  271,  291. 

Tetrads  (Vierergruppen),  179;  origin,  186 
in  Ascaris,  187  ;  in  arthropods,  188-193 
ring-shaped,  188;  in  amphibia,  191,  192 
origin  by  conjugation,  199  ;  formulas  for 
186,  193,  200,  201. 

Thalassema,  fertiHzation,  143  ;  centrosome, 
228 ;  attraction-sphere,  235. 

llialassicolla,  experiments  on,  250. 

Tonoplast,  37. 

Toxopneiistes,  cleavage,  8;  mitosis,  76; 
ovum,  91 ;  spermatozoon,  99 ;  fertili- 
zation,    146;      paths      of     germ-nuclei, 


152;  polar  bodies,  174;  double  cleavage, 

260. 
Trachelocerca,  diffused  nuclei,  26. 
Trophoplasm,  301. 
Tuhidaria,  regeneration,  293,  325. 
Tunicates,  egg-axis,  280;  cleavage,  281. 

Unicellular  organisms,  3  ;  mitosis,  62 ;  con- 
jugation, 163;  reduction,  198;  experi- 
ments on,  248-252. 

Unio,  cleavage,  272. 

Vacuole,  37. 

Vanessa,  ovarian  egg,  115. 
Variations,  9  ;  origin  of,  329,  330. 

Vaucheria,  membrane,  251,  254. 
Vitelline  membrane,  96 ;  of  egg-fragments, 
97;   formation  of,  146;   function,  148. 

Volvox,  germ-cells,  89,  98. 

Vorticella,  conjugation,  167. 

Yellow  cells  (of  Radiolaria),  37,  2H. 
Yolk  (see  Deutoplasm),  90,  94. 
Yolk-nucleus,  115,  1 18. 
Yolk -plates,  94. 

Zwischenkorper  (mid-body),  52. 
Zygnema,  membrane,  252. 
Zygospore,  169. 


Columbia  University  Biological  Series. 

4' 

EDITED  BY 

HENRY    FAIRFIELD    OSBORN, 

D(i  Costa  Professor  of  Zoology  in  Columbia  University. 


This  series  is  founded  upon  a  course  of  popular  University 
lectures  given  during  the  winter  of  1892-3,  in  connection  with 
the  opening  of  the  new  department  of  Biology  in  Columbia 
College.  The  lectures  are  in  a  measure  consecutive  in  charac- 
ter, illustrating  phases  in  the  discovery  and  application  of  the 
theory  of  Evolution.  Thus  the  first  course  outlined  the  de- 
velopment of  the  Descent  theory;  the  second,  the  application 
of  this  theory  to  the  problem  of  the  ancestry  of  the  Vertebrates, 
hirgely  based  upon  embryological  data;  the  third,  the  applica- 
tion of  the  Descent  theory  to  the  interpretation  of  the  structure 
and  phylogeny  of  the  Fishes  or  lowest  Vertebrates,  chiefly  based 
upon  comparative  anatomy  ;  the  fourth,  upon  the  problems  of 
individual  development  and  Inheritance,  chiefly  based  upon  the 
structure  and  functions  of  the  cell. 

Since  their  original  delivery  the  lectures  have  been  carefully 
rewritten  and  illustrated  so  as  to  adapt  them  to  the  use  of  Col- 
lege and  University  students  and  of  general  readers.  The  vol- 
umes as  at  present  arranged  for  include: 

I.  From  the  Greeks  to  Darwin.    By  Henry  Fairfield 

OSBORN. 

II.  Ampliioxus  and  the  Ancestry  of  the  TertebrateSo 

By  Arthur  Willey. 

III.  Fishes^  Living  and  Fossil.    By  Bashford  Deait. 

IV.  The  Cell  in  Development    and    Inheritance.     By 

Edmund  B.  AVilson. 

Two  other  volumes  are  in  preparation. 


THE   MACMILLAN   COMPANY, 

66   FIFTH   AVENUE,  NEW   YORK. 


I.    FROM  THE  GREEKS  TO   DARWIN. 

THE  DEVELOPMENT   OF    THE  EVOLUTION   IDEA. 

BY 

HENRY   FAIRFIELD   OSBORN,   Sc.D.,  Princeton, 

Da  Costa  Profenwr  of  ZoMorjn  hi   Coliinibia   University. 
8vo.    Cloth.    !52.oo,  net. 


This  opening  volume, "  From  the  Greeks  to  Darwin/^  is  an 
outline  of  the  development  from  the  earliest  times  of  the  idea  of 
the  origin  of  life  by  evolution.  It  brings  together  in  a  continu- 
ous treatment  the  progress  of  this  idea  from  the  Greek  philoso- 
pher Thales  (640  B.C.)  to  Darwin  and  Wallace.  It  is  based 
partly  upon  critical  studies  of  the  original  authorities,  partly 
upon  the  studies  of  Zeller,  Perrier,  Quatrefages,  Martin,  and 
other  writers  less  known  to  English  readers. 

This  history  differs  from  the  outlines  which  have  been  pre- 
viously published,  in  attempting  to  establish  a  complete  conti- 
nuity of  thought  in  the  growth  of  the  various  elements  in  the 
Evolution  idea,  and  especially  in  the  more  critical  and  exact 
study  of  the  pre-Darwinian  writers,  such  as  Buffon,  Goethe, 
Erasmus  Darwin,  Treviranus,  Lamarck,  and  St.  Hilaire,  about 
whose  actual  share  in  the  establishment  of  the  Evolution  theory 
vague  ideas  are  still  current. 

TABLE    OF    CONTENTS, 
I.  The  Anticipation  and  Interpretation  of  Nature. 

11.  Among  the  Greeks. 

III.  The  Theologians  and  Natural  Philosophers. 

IV.  The  Evolutionists  of  the  Eighteenth  Century. 
V.  From  Lamarck  to  St.  Hilaire. 

VI.  The  First  Half-century  and  Darwin. 
In  the  opening  chapter  the  elements  and  environment  of  the 
Evolution  idea  are  discussed,  and  in  the  second  chapter  the  re- 
markable parallelism  between  the  growth  of  this  idea  in  Greece 
and  in  modern  times  is  pointed  out.  In  the  succeeding  chap- 
ters the  various  periods  of  European  thought  on  the  subject  are 
covered,  concluding  with  the  first  half  of  the  present  century, 
especially  with  the  development  of  the  Evolution  idea  in  the 
mind  of  Darwin. 


II.    AMPHIOXUS  AND  THE  ANCESTRY 
OF  THE  VERTEBRATES. 


BY 


ARTHUR   WILLEY,    B.Sc.    LOND., 

Tutor  in  Biology,  Columbia  University ;  Balfour  Student  of  the 
University  of  Cambridge. 

8vo.    Cloth.    $2.50,  net. 


The  purpose  of  this  vohime  is  to  consider  the  problem  of  the 
ancestry  of  the  Vertebrates  from  the  standpoint  of  the  anat- 
omy and  development  of  Amphioxus  and  other  members  of  the 
group  Protochordata.  The  work  opens  with  an  Introduction, 
in  which  is  given  a  brief  historical  sketch  of  the  speculations 
of  the  celebrated  anatomists  and  embryologists,  from  Etienne 
Geoffroy  St.  Hilaire  down  to  our  own  day,  upon  this  problem. 
The  remainder  of  the  first  and  the  whole  of  the  second  chapter 
is  devoted  to  a  detailed  account  of  the  anatomy  of  Amphioxus 
as  compared  with  that  of  higher  Vertebrates.  The  third  chapter 
deals  with  the  embryonic  and  larval  development  of  Amphioxus, 
while  the  fourth  deals  more  briefly  with  the  anatomy,  embryology, 
and  relationships  of  the  Ascidians;  then  the  other  allied  forms, 
Balanoglossus,  Cephalodiscus,  are  described. 

The  work  concludes  with  a  series  of  discussions  touch- 
ing the  problem  proposed  in  the  Introduction,  in  which  it  is 
attempted  to  define  certain  general  principles  of  Evolution  by 
which  the  descent  of  the  Vertebrates  from  Invertebrate  ancestors 
may  be  supposed  to  have  taken  place. 

The  work  contains  an  extensive  bibliography,  full  notes,  and 
135  illustrations. 

TABLE   OF    CONTENTS. 

Ixtroductio:n^. 

Chapter    I.  Anatomy  of  Amphioxus. 
11.  Ditto. 

III.  Development  of  Amphioxus. 

IV.  The  Ascidians. 

V.  The  Peotochokdata  ix  their  Relation  to 
THE  Problem  of  Vertebrate  Descent. 


III.    FISHES,    LIVING   AND   FOSSIL, 

AN  INTBODUCTOBY  STUDY. 

15  Y 

BASHFORD   DEAN,  Ph.D.,  Columbia, 

Instructor  in  Bioloyy,  Columbia  University. 
8vo.    Cloth.    $2.50,  net. 


This  work  has  been  prepared  to  meet  the  needs  of  the  gen- 
eral student  for  a  concise  knowledge  of  the  Fishes.  It  contains 
a  review  of  the  four  larger  groups  of  the  strictly  fishlike  forms. 
Sharks,  Chimaeroids,  Teleostomes,  and  the  Dipnoans,  and  adds 
to  this  a  chapter  on  the  Lampreys.  It  presents  in  figures  the 
prominent  members,  living  and  fossil,  of  each  group;  illustrates 
characteristic  structures;  adds  notes  upon  the  important  phases 
of  development,  and  formulates  the  views  of  investigators  as  to 
relationships  and  descent. 

The  recent  contributions  to  the  knowledge  of  extinct  Fishes 
are  taken  into  special  account  in  the  treatment  of  the  entire 
subject,  and  restorations  have  been  attempted,  as  of  Dinichtliys, 
Ctenodus,  and  Cladoselache. 

The  writer  has  also  indicated  diagram matically,  as  far  as 
generally  accepted,  the  genetic  relationships  of  fossil  and  living 
forms. 

The  aim  of  the  book  has  been  mainly  to  furnish  the  student 
with  a  well-marked  ground-plan  of  Ichthyology,  to  enable  him  to 
better  understand  special  works,  such  as  those  of  Smith  Wood- 
ward and  Giinther.  The  work  is  fully  illustrated,  mainly  from 
the  writer's  original  pen-drawings. 

TABLE    OF    CONTENTS. 

CHAPTER 

I.  Fishes.  Their  Essential  Characters.  Sharks,  Chimaeroids,  Teleo- 
stomes, aud  Luug-lishes.  Their  Appearance  in  Time  and  their 
Distribution. 

II.  The  Lampreys.     Their  Position  with  Reference  to  Fishes.      Bdel- 
lostoma,  Myxiue,  Petromyzon,  Palaeospondylus. 

III.  The  Shark  Group.     Anatomical  Characters.     Its  Extinct  Members, 

Acauthodiau,  Cladoselachid,  Xenacanthid,  Cestracionts. 

IV.  Chimaeroids.     Structures  of  Callorhyuchus  and  Chimaera.     Scjualo- 

raja  and  Myriacanthus.  Life-habits  aud  Probable  Relationships. 
V.  Teleostomes.  The  Forms  of  Recent  "  Ganoids."  Habits  and  Dis- 
tribution. The  Relations  of  Prominent  Extinct  Forms.  Crosso- 
pterygians.     Typical  "  Bony  Fishes. " 

VI.  The  Evolution  of  the  Groups  op  Fishes.  Aquatic  Metamerism. 
Numerical  Lines.  Evolution  of  Gill-cleft  Characters,  Paired  and 
Unpaired  Fins,  Aquatic  Sense-organs. 

VII.  The  Development  of  Fishes.  Prominent  Features  in  Embryonic 
and  Larval  Development  of  Members  of  each  Group.     Summaries. 


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